I have run across 2 of them over the years,, and part of the reason I joined here is to network with the knowledge base about a possible 'nother one or variation.
They are super rare, Myles Raymond of Scotland had on his old website some great material and many years ago I corresponded frequently with him researching these rare beasts.. at that time not a lot of info out there. I used to have 2 plunger A10s (now one) and at one point was going to build one of them as a "BSA HotFlash" tribute bike.
I also had a super rare correct Superflash gas tank (petrol) that came from Australia. About 7-8 years back I saw a SuperFlash being sold on Feebay and it was missing the tank. In a fit of remorse it seemed bad Karma that I had one and that bike did not. So, I contacted the seller. He connected me with the buyer on FeeBay who was very pleased to hear from me.
I waffled a bit as we both knew how valuable the tank was,, An auction on eBay would have probably been well attended. I also had <Cough> Material needs as well.. I did not want to take advantage either. In the end I think we made a fair deal and the buyer got my tank. Perhaps that guy is on here? Would love to know how it worked out.
(The tanks for the Plunger SuperFlash differ slightly, the mere mortals and general public would NEVER know the difference, but it is a significant detail to get right)
Here is 2 pix i slurped recently researching another possible Superflash.
I also have some rare literature from Hap Alzina I might scan if theres interest.