Got all excited when I first read the add as the box of bits that might end up being G14 slowly gets bigger.
1948 did not phase me as lots of people get years wrong, then they uploaded the pictures.
They say the frame number is G14 *****
Matchless ? but it looks way to big for a 250 or 350
http://www.gumtree.com.au/s-ad/kings-langley/motorcycle-scooter-parts/bsa-g14-1948-frame/1134215321
I'm no expert on AJS/Matchless (or anything else!) but that is a circa 1964 AMC (AJS/Matchless) twin frame, and probably the G15 variant (going by the number) which was mostly made for the US market. The G15 was a stretched (750cc) version of the previous sports twins and included the beautiful "desert sled" models made for desert racing in the US Western states.
REgards, Mark