Pewi: Contact the people at Ithaca before you shoot steel. My 37 was made in the late 60's. I shot # 2 steel in it and ringed the barrel. An Ithaca pump made in the 50's is too nice to take a chance with. I would recommended Bismuth of some other soft nontoxic, but check with Ithaca to be sure. They are now in Ohio.
All the best.