This was probably a "Convertible" model. Produced in .357mag/9mm and 45 Colt/45APC. Since the frame IDs it as 45 Caliber, it would be the interchangeable cyclinders installed that would define actual chambering. Normally there is a "S" (size varies) on the front of the cyclinder and the last four digits of the Ser# for the correct cyclinders to frame match. I don't know of calibers ever being marked on Convertible gun cyclinders. The difference between in bore size between the 357/9mm and 45Colt/45APC should be obvious.
If the inside cyclinder bores are smooth: it should be 45Colt, as it head spaces off the case rim. If each cyclinder bore has a internal ridge ring: it is for head spacing off the .45ACP case mouth.