If it's a 1957 gun, it's likely a 409. Remove the forend; you should find "409" marked on the underside of the wood. (Silverhawks were essentially 409's with some modifications for the American market, and were in fact marked 409 on the forearm wood.)

Above picture with the square crossbolt gun looks odd to me. If the crossbolt is square--which it certainly appears to be--then why is the hole in the rib extension round, like for a Greener? Square peg in a round hole???