As said above the number should be STAMPED into the frame.
Anything in raised lettering/numbers is a casting number.
The blurb about finding the frame number is a little unclear.
It sez up to 1957 its under the seat, and from 1958 its up front.
Your A prefix number fits into the series of yearly numbers listed
for 1956/1957 in
Roy Bacons Ajay and Matchy resto book.
You G80S should look something rather like this for 1957.
https://i.ebayimg.com/images/g/xjMAAOSwfZxgvTIu/s-l1600.jpgThe 350cc and 500cc versions were near identical, except for the engine capacities.
Minor details could vary a little for specific markets.
Hopethishelpsalso.