There are quite a few strange iterations of Airsporter stocks. Some of it probably occurred owing to lumps dropping off the 3 piece laminations, possibly splitting and something else being added/modified.
Don Blocksidgege did a custom special limited run.
As much as some might admire Dons work, it looked a bit of a contrivance.
It did include an added cheek piece section so I wonder if you might have one of these.
He had similar works which missed the mark on an FWB Sport but some of his engraving work on Webley pistols was excellent. A far better engraver than he was a stock maker.