This has come up on eGun, the seller doesn't know what it is, and I think it's a rare thumbhole stock (lochschaft) for an LG55. It's a bit of a slab but might be worth a punt if the price doesn't go skywards.
http://www.egun.de/market/item.php?id=5697459
Vintage Airguns Gallery
..Above link posted with permission from Gareth W-B
In British slang an anorak is a person who has a very strong interest in niche subjects.