Thank you very much for the responses, will have a closer look at mausers for sure!
To answer the question about the white lines, it just looks like some white paint transfer from rubbing against a white door or something similar.
The wood is one piece and very healthy, no gluing or chunks missing etc. certainly a good candidate for adding a barrelled action to!
Would really like to get to the bottom of this, as the stock is in good shape and if I can find out exactly what fits in there I would seriously consider filling it so to speak, I think it would be a great one of a kind. i do have a few other display items that are also functional, would be nice to add this piece to the "lot"
Will keep checking but please keep the guesses coming as I want to be sure before I commit to buying something that doesnt fit.
Best regards,
Coldrider