"At what point did manufacturers start puttting Serial #'s on their guns?"
It varied WIDELY from maker to maker, and some makers put serial numbers on SOME of the guns in their lineup while other models made during the same time frame were not serial numbered.
Couple of cases in point...
Post World War II Remington serial numbered their bolt action centerfire rifles, but didn't serial number their mid- to high-end .22s such as the 521-T.
Savage serial numbered its Model 99 lever action, but didn't serial number its Model 219, a single shot rifle.
Changes in the law in 1969 mandated that ALL firearms be serial numbered with unique numbers. Prior to that time, different models would often share the same serial number. Smith & Wesson was famous for that.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.