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M17? As in sported M1917?
Its going to depend on who did it, and how the receiver was contoured.
If it is a BSA conversion, Remington 700 bases might work.
Three holes would suggest a bridge mount.
As a guess, I would suspect a M700 mount might be the one, if the rear bridge has been contoured like a 700.
Is there a gunshop/gunsmith close by, where you could try some bases against the receiver?