650SS wrong gearbox

The mainshaft doesn't have a 4th gear. What it does have is the sleeve gear (that has the drive sprocket attached) Top gear (4th) is direct drive (1 to 1) through the box as that has the least transmission losses. Even if you did swap gears about (and good luck with finding them!) you would notice little-if any difference in the lower gears, top gear would be the same. 
Worry about something else, like how to get 1960's air in the ancient original tyres.         

Hello Gary - the 650SS (and Atlas) used a gearbox with the number pre-fix "NA" whereas most of the slimline Dominators had a pre-fix "N".  As you quite rightly say the 650SS ratios are commonly known as AMC2 set.  The layshaft 4th gear had 23 Teeth rather than the previous 24 Teeth. This raised the ratio of the first three gears and reduced the large gap between third and fourth gear.  The raised gearing of bottom gear wasn't a problem when pulling away because of the increased power output.  Obviously swapping the gearing from AMC1 (pre-fix N) to AMC2 (pre-fix NA) requires the pair of gears to be changed together.  As well as the gear ratio change, the kickstart spring on the NA gearbox is much heavier and more reliable. Unfortunately the springs are not interchangeable.  The kickstart shaft, inner and outer covers must be swapped to use the heavier spring.  I suggest you have a read of the Dominator Service Notes on the website library particularly Chapter 10.  My notes here summarise the salient points of your query from the DSNs. If you do swap the gear pair you can fit Atlas, Mercury and early Commando ones.  Additionally all the Dominator bikes built at the Matchless factory in Plumstead had the NA gearboxes.  Good luck, Howard   

Howard has miss quoted the Layshaft 4th gear tooth count. It is the Sleeve gear that has 23 teeth, (or 24T if the earlier box) the layshaft 4th gear has 18 teeth (to match the 23 tooth sleeve gear) for the later AMC box
Looking at the gear chart on page 134 of the Heavy twins compendium it has the sleeve gear @24T and the Layshaft 4th @18T
AN supply two types of 18tooth layshaft 4th gears, this is because the tooth form is slightly different, one of them is to engage with the 24 tooth sleeve gear.   .
 

Howard is spot-on with his information and suggestions.  The Gearbox section of the DSNs is packed with facts, figures and useful maintenance hints.

Adding a few more important notes to this thread.
The Norton 650 road bikes with NA gearboxes first appeared from May 1962 onwards.  So these would be Bracebridge built 650s. The first being 18  103021 with AMC 2 box NA 1044.  
But typical of Norton production  650 bikes with boxes stamped with lower NA numbers were dispatched later. The lowest recorded being NA1003.  The majority of the first 1000 NA stamped gearboxes heading off to decorate Manx Racer frames.

As 650 production began with bike 18 93601 there were quite a number that left the Bracebridge  factory with an AMC 1 box.  Possibly over 1500.
Even the first Atlas bikes were fitted with AMC 1 boxes. The first Atlas kitted with an AMC 2 box being 20 103639 in July 1962.

Surely this is a case of not 'what was originally fitted' but more like 'what is currently fitted' is it a 650SS or Cafe racer, the latter could have a variation of the parts fitted from whenever. 
If I had a pound for everytime someone had rang me saying that I have sent the wrong part to them I would have retired ages ago. The call usually runs along the line of 'I sent the part for a 19xx Norton Dominator' but then get the reply 'it's actually a Cafe racer / Triton etc' usually followed by it's an original cafe racer or Triton.  

The Dominator Service Notes table of gear teeth appears to be incorrect for AMC2.
The mainshaft sequence is listed
21, 18, 14, 23.  
Other publications list
23, 21, 18, 14

Well spotted Dave.  Definitely a rogue number in the list.
The final 23 should prefix the queue of mainshaft digits and actually read......
23   21   18   14

Very strange , i was sure my 99  assembled in 1959 has an AMC2 box.  And also my spare box of similar age  Going to have to take one  down to check.

As Ashley says......there is no guarantee what you will find jnside a dismantled gearbox. 
The Gearbox records are pretty accurate but even these are prone to Factory fiddling. It does not take too much engineering to change the ratios of both AMC 1 or AMC 2 gearboxes to assemble a close ratio set for Clubman racing. 
Bracebridge assembled a good number of Special Sports Dominators with twin carbs, polished innards and possibly special gearbox gear ratios.

I converted my own AMC 1 box to  AMC 1 1/2 by dropping the sleeve gear down a tooth to 23. Easy to do and consequently making a huge improvement to the lower ratios.
The photograph shows the starting point being a box of bits.