Culit, date stamps weren't used until the 1912 proof law, and even then only sporadically. Reading 1 3 8 as 1. March 1908 is like a coin stamped 20 BC. Also, the B U series of proofs included smokeless powder, there isn't always a separate N stamp if you have BUS (or BUG on rifles) on the gun.
F 135 - the right choice