the bike with the AJS engine m, what type of ajs engine is that, it has a different barrel and rocker cover set up than the topical G3L,
also note the horn behind the engine and regulator box in front of the carb. the suspension units look like a cut down version of the front forks, wonder if they still have the same spring/damper and oil set up, note oil drain screws.
That head is interesting, seems as if the whole engine was built out of odds and ends.
It is BS that it signifies a 500cc, it shares feature with the G90 prewar head but looks to me to be offthe hicam Sunbeams made late 30s when Sunbeam owned by Matchless, who also did a 250 and 350 version. B series.