.22 Bolt.

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If it's the one that's been on Naturabuy for a while; I think you'll find it's for a .22 No5 trials rifle. I had a look at that and after searching a bit in Skennerton's book and posting on this sight, that's what we came up with.
Now you've got to find one of the 105 built and you'll be a happy bunny! Does it have any numbers on it?
It's certainly interesting to find out how it ended up in France