Mick i reckon both bikes (as in the one i posted and the Gidgee one on the trailer
here) are defo pre-unit B series BSA singles as the earlier Empire Star had vertical fins on the head not horizontal ones, though the motors are very similar. no idea what the front end is on either bike but you'll notice the bike pictured on the Trailer has a stock looking B series ridgid frame where as the
widowmaker has what i think is a modified frame in the rear, the
widowmaker gearbox looks like the rigid/plunger model box and same on the Gidgee bike, they could be the same bike but i do know these cast iron B's were pretty popular as speedway bikes because they were a lot cheaper then the Alloy Goldstars and the Goldie cranks, pistons, valves, springs and cams were a straight swap.
JAP i'll email the club and see if they can shed some light, cheers.
-Nick