I suggest that you click on records on this site. This will give you basic information and who to contact. I doubt very much that you will recover the original number but to have some chance you will need as a minimum the buff log book. The problem you then have is that it is listed as a Triumph and no inspection will support that. if the majority of the main components can be verified as Norton you may be in with a chance of an age related plate. Triton's are now proving problematical to get past the DVLA. I would not advise trying to deal with the DVLA yourself.
Definitive advice will be available through the records officer

The DVLA might offer a Q plate, if this is the case then check you can insure it. You may find it very hard to insure these days. Why not just apply for the V5 under the information that DVLA have as it will be far easier and if converted back to a Norton of the period a far more worthwhile bike.  

Hmmm Where to start!
I used to be the registration help for a classic ford club and worked at DVLA for a few years, I know the rules have a changed a bit but here are my suggestions:

You have a few options - throw yourself at DVLAs mercy and tell them what you have, but I'm pretty certain that way will get you a Q plate

Secondly you could treat it as a barn find and put a Norton engine in it, providing its still a Norton sufficiently for the club to authenticate it for you (engine wheels gear box forks etc ( have a word with the records/registration officer about what he'd want to see), and apply for an age related mark, it may need to be inspected so it can't just be a paper excercise. Once given a new age related mark you will get a V5 and you can put the Puma back in and notify a change of engine. A change of engine is usually straight forward. 

Lastly you can search for the old registration ledgers and see what the reg mark was first registered as and also ask DVL for a full vehicle history - they used to give these without a problem but due to Data protection/GDPR you need to show good cause for the information and all you may do is alert them to the problems!  My guess is that it started as a Treiumph, then a Triton then a Norton/Puma, or someone just stuck a plate on it that they had a V5 for! 

Finally (I know I've already said lastly!) but which ever route you take you need to be open with your insurers and tell them its been modified. I've found Bikesure the best for one of my modified bikes. 

Giood luck - Dan 

In fact his Option 2 was more or less what I did when I built a Tri-Greeves a year ago. Built it as a Greeves with a Villiers engine, had it certified by the Greeves Association to register it as a Greeves (age related) then popped the Triumph engine in and notified DVLA of change of engine capacity and engine number. Of course being a technical ignoramus I had no idea it was a Triumph engine! Peter James and RH were both happy to cover it on normal terms.

My current bitsa is a BSA frame with a Suzuki DRZ400 engine (ex-CCM) - not built by me. It's registered as a BSA on what I assume is an age related plate. Oddly enough the previous owner was having it MoT'd every year but I intend to submit a V112 form for exemption once that MoT has run out. They can only say no. In fact given the heritage of CCM from BSA I could argue it was a logical progression. But maybe not. Again Peter James are quite happy to cover it.

Ian

This is the guidance, probably open to interpretation! Not sure how an engine change changes the way a bike works?! 

Vehicles that do not need an MOT

You do not need to get an MOT if:

• the vehicle was built or first registered more than 40 years ago
• no ‘substantial changes’ have been made to the vehicle in the last 30 years, for example replacing the chassis, body, axles or engine to change the way the vehicle works

... but the guidance specifically says "other than for motorcycles" - but then makes no further reference to bikes! As the engine isn't part of the MoT other than checks for leaks etc I can't see how changing it makes any difference. When I built the Tri-Greeves I used the V112 form and there were no problems.

Ian
I stand corrected! I only read the summary which doesn't mention excluding motorcycle, the full guidance does, which doesn't help!
Looks like you are ok then!
Dan 

.... but it's another piece of badly drafted quasi legislation. I can't imagine anyone would be too bothered though. I'll let you know if I end up in the dock!

Ok, Ive got to the bottom of it i think, the Statutory instrument(which secondary rather than quasi legislation! :-)   says

Search

The Motor Vehicles (Tests) (Amendment) Regulations 2017

Vehicle of historical interest” means a vehicle which is considered to be of historical interest to Great Britain and which—

(a) was manufactured or registered for the first time at least 40 years previously,

(b) is of a type no longer in production, and

(c) has been historically preserved or maintained in its original state and has not undergone substantial changes in the technical characteristics of its main components,

so the question then is how is "substantial" defined, and as it stands, it was apparently deliberately not defined - the Minister said in the press release  (which i now cant find!) that it was deliberately undefined so that they could be flexible in the guidance. 

If i can find the link to the consultation response again, ill post it here.

Here it is, see page 7. https://assets.publishing.service.gov.uk/media/5a820336e5274a2e8ab57088/government-response-to-exempting-vehicles-of-historical-interest-from-roadworthiness.pdf

Ian
I appreciate that you've prob done research already but I've spelled it out for anyone else who may search on it in the future. 

Dan 

If it's on DVLA's computer record then surely you can just write to them and apply for a duplicate V5C ? They will write to the last known address and give them a period to reply. If no response then a new record of registered keeper will be sent to you.  Effectively, you didn't receive a V5C with the purchased vehicle. You can't do it on-line, but otherwise it's a common procedure.

Whether a V5C stating "Triumph" will be sufficient for a Featherbed special in the future is another question.

Up until the 1980s, there was no list of approved manufacturers. Any "Special" builder could call his bike whatever he wanted.

you've delved further than I have and I look forward to reading that report in its entirety. Meanwhile wing and a prayer will be my approach.....

... the report dates from 2017 and they still haven't managed to tell us the criteria for bikes. Obviously not a priority then.