I suppose the obvious suspicion/hope must be a connection to Frank Clarke. There’s something about that breech seal arrangement, and we know he was keen on concentric designs. Assuming it is indeed factory made and not a brilliant apprentice piece, and that does seem a reasonable assumption, you need a creative british airgun designer with an interest in concentric pistols who might have produced a piece leaving no known written records

Put like that FC would seem to be a reasonable theory at least.