Which engine.

If a 1973, it's unlikely to be. Interpols never were from new...but the engine could have been rebuilt to any spec. What size are the carbs ?

Both 1972 MkIV and 1973 MkV have the features that you describe and a relatively small number were Combats

I too have an ex Interpol 750. Because of the extra work they did to convert a standard model into a Police bike there is often a big gap between the build date and the headstock plate date. For example mine was built in January 72 but wasn’t registered until August 72.

So your 73 could easily have a build date in 72. Greg Marsh’s survey would suggest that yours is from around July/August 72.

https://www.gregmarsh.com/MC/Norton/CommandoSurvey.aspx  

The other check is to look on your cylinder head in front of the head steady for the letter C, denoting combat.

The breather on the back of the cases and the small oil drain plug does not mean Combat, just that the bike was from around the 1972 timeframe (build date, not registered date) The two easiest checks are for the "C" on the head (which you can also confirm by the smaller than normal gap between the barrel and the head, denoting it was heavily skimmed. Also look at the cam lift - you can have an easy glance at this without tearing down the engine by slowly turning the motor over without the rocker covers on.

• A standard cam has a negligible difference between inlet and exhaust valve lift (8.432mm 'vs' 8.178mm)
• The Combat cam has noticeably more inlet valve lift (9.906mm 'vs' 8.788mm)

It is by no means scientific are wholly accurate, but by holding a dial indicator or even a vernier caliper on the top of each valve adjuster while, with the plugs out your friend slowly turns the back wheel in gear, it is a great way to see if your cam is standard or not.

https://www.nortonownersclub.org/history/commando

The Commando was now settling down after all the warranty claims from the ill fated Combat (discontinued from engine No. 211110)

So just within the range of it being a possible Combat, Grants checks will sort out the answer. At least if it is a Combat it would have left the factory with superblends. 

Engine number means its a 72 engine not a 73 which started at 220000 but a late 72 which had some crankcase mods eg to stop the rear engine mount cracking, and if later fitted with a high compression head then it was not originally a Combat.

http://atlanticgreen.com/engcases.htm

72 and 73-750's all used the same "combat breather" cases until the end of production. The early cases had the machining for starters done, but as later 20M3S the hole was blocked off and unused. These early cases had been poorly machined with a sharp cutter as well as being cut to deep into the casting itself. This stress riser contributed to many cracked cases as a result of over revving the "combat state of tune" engines. Note how large the red outlines area is and compare to the later cases (below) to see how much less had been cut. The green line above the rear engine mount is where a typical crack might propogate. This style of finish machining stopped in the 206xxx to 208xxx sn region.

With the High Compression head and a hi lift cam then it may be currently tuned as a Combat but did not start life as a Combat.

Hi, I have just purchased 211778 which I believe was built in 9/72 as an Interpol. It has been in bits for 7-8 years. It's hard to know what is original but it has 32mm, a skimmed head ( no C stamp). The bottom end has been built by a reputable classic car engine specialist and I will probably strip it again.  Is it fair to assume she was born 'non combat' please?

No C = not a Combat. You can convert a std head to Combat specs by skimming the head and opening out the inlet ports from 30 to 32mm.