Hi Tiller,
As Jeff says, the early A7 long stroke engine was quite unique. It was only produced from 1947 to 1950 and although it looked very similar, it was completely different to the later 1951 short stroke engine.
If you want more information on it there is a good write-up on the A7 long stroke models on the BSAOC website. Check out
www.bsaoc.demon.co.uk/earlya7.htm where you will also see a good picture of one.
The squareness of the bottom of the timing case and the lack of the 3 rifles logo will tell you if the early engine is fitted.
If you do decide to get it and you need parts for it make sure that you always ask for parts for the "Early Long Stroke A7" model. Even though they look similar, very few parts (if any) are interchangeable between the two types of engine.
Beezageezauk.