In my opinion this is one of the most interesting & intrigueing threads for a goodly while. There are many good points raised & although we may not know for certain, at the moment any rate, whose design this is or who made it makes for a fascinating read.
Garvins point about the trigger guard spring & cocking lever arrangement being such that they might not infringe other patents seems valid to me.
Was it a BSA prototype? I really don't know but if it was then I wonder why it never went into production. Even if it wasn't BSA who oversaw the design & the making of this example I still wonder why it didn't get marketed. Was the competition too well established, did the outbreak of WW2 effect it, was it deemed too tricky to make, would the manufacturing cost be too high & consequently the selling price be too high too?
As an aside I know not all speculative ideas/items get patented, there are various reasons & sometimes it's because it attracts attention to what a company is looking at. Not saying this is the case with this but maybe it's a possibility. Either way it's a bit of a shame this didn't get into production, I'd have been on the look out for one if it did. Such a neat design, cleverly thought out, I really like it & am quite taken with it.