On the older '35's the serial No. was often on the back block. You can date the '35 often by the configuration of the safety. Up to 1976 (ish) they had no safety. '76 to '80 they had single sided safety and from then on the current double sided safety. Up to 1980 the '35 also sported a leather piston seal. Work wise, it's up to you and how much you want to spend (or do). You might want to check under the silencer adaptor as the pressed in dove tail for the front sights would often require filing off for an adaptor to fit. If the plunger doesn't hold the barrel locked, it could have worn (or the spring could be sha99ed) - continued use will wear a groove in the breech face of the cylinder. Re-bluing and other finish options (Cerakoting) weigh up about the same cost so just decide how much you want to do with it.