Malvin,
It is clearly broken,not 3 part. The marks show it was proofed for shot(S),is choked (W),and was given a view proof(U)which is a close inspection. The chamber is 16ga,65mm and the barrel is 16 ga ahead of the chamber. The rifle proofs are not clear, but it has already been IDed as 6.5x52R aka 25-35 Winchester.
Mike