The closest SN on my list to your 303123 is 306710 which shipped on 7/15/1919, also a 5" Military and Police (M&P) model. Yours would probably have shipped close to that date. Even though it was made before heat treatment of cylinders began, that is of no practical consequence regarding ammunition that can be used. Any standard velocity .38 Special ammunition sold today can be used safely. It is probably not a good idea to fire +P ammunition, but even shooting some of that will present no hazard.