You likely have a convertible model. measure the headspace on cylinder number 2 and see if it is only as deep as a 9mm. You could do this by attempting to insert a .357 snap cap or other dummy round. If that didn't go in far, you could then attempt a 9mm dummy round. The second cylinder of a convertible model would look identical outside to the .357 cylinder. However, the chambers are only cut as deep as the mouth of a 9mm cartridge.
If you have a convertible, you can enjoy some near zero recoil and cheaper shooting with 9mm and not worry about loading the front of your .357 chambers with crud from shooting .38 spcl.