They who?
Smith & Wesson started putting serial numbers on their guns in 1857, Model No 1, First Issue, serial no 1.
Colt had about a 20 year head start with the Paterson family in 1837, several models each with its own serial number range starting at 1.

There are a lot of minor brands and imports without serial numbers until required by law in 1968. But it is illegal to remove the serial number from a gun that ever had one, or to even own a gun with defaced serial number.