Hi Brian, ya I figured there were. Interestingly the known experimental frames assembled with wooden grips as well.

I have yet to find a lightweight frame that this matches in design, other than the Inglis ones. However, I really cannot believe given their extreme rarity that this is one. However, this example of an unfinished, cast frame from Clive Law's collection does have the same squared off locking shoulder pin hole as mine (top left of image). I am going to try and get some history on mine, but chances of that are slim as it came from an auction.

Browning Hi Power Lightweight Project Items