The image uploaded has an error as I can't view it.
The lower blade plate hub taper I thought was the same ,sometime there is a slight difference in machining tolerances so the hub may fit on a little more or less than another mower but hardly measurable.
Some people say around 107,000 and on, the font of the number stamp is smaller and that they are model 3.
Also seen a few 2a mowers in the 68,000 range.
The 2a has different numbers on the conrod and the 2a and early model 3 has smaller rectangular transfer ports compared to later round transfer ports.
It depends how original you want the 2A build to be as some people want everything original in the motor but personally as you can't see the internals, it doesn't make much difference , just use the right crankcase number.
The 2A has the circlip top carby instead of the later wire clip top also uses the thin long fuel tap and a black plastic throttle control.
The image of the 2A below is from the Vintage Mowers Site.
