Model Numbers

"The parts list tells us the 650/99 had two carburettors like the 650SS, but they have different part numbers. "

IAW the parts book #:  http://atlanticgreen.com/nhth.htm

650 only had S650/136 downdraft head.

99ss had 88ss/136 head and  splayed log manifold for dual carbs

The greater majority of 650 Nortons built and sold up to October 1961 were the Manxman 650 model. This version had a very individual styling that included a small peanut shaped tank, a special exhaust and silencer system, high bars, Red seat  and an everything else in a Blue paint scheme. Although a good many of these Manxman bikes headed off to the USA,  a fairly significant number from the first 300 were also  shipped to Sweden, Austria, France and Italy. 

Not everybody cared for the US specification and by February 1961, exports to Europe included orders for the bike to have the standard Model 99 styling.  Including the larger 99 Tank, Black Dual Seat and 350 x 19 Rear Wheel (instead of the US 18"), Along with standard bars and Dominator exhaust systems.

These Nortons were designated as the 650/99 model which appears in the Amal and Lucas spare parts books from that year on. In essence they were an early variation of the 650SS but that name was being kept secret for its big launch in October 1961. 

For some reason the main question is ignored.... carb differences?

carb induction changes IMO relates to drastically different heads and manifold style account mainly for different carb tune.

Model name, color, or sales market seems a weak argument for different carbs...?

IIRC 61-on, all NHT had the same 22729 cam

PS214 pg 54-on has all the particulars for 61-62 99ss carbs.

I would not worry about the Bacon book. Nice guy(RIP), I met him and chatted at length about our norton experiences and involvement .

Would you be so kind to share the norton parts book #   that shows the 99 in 64

I would like to research and see it.listed

thanks