By the serial number it's neither a 1st nor 2nd Generation. The 1st Gen's were serial numbered as follows. The military contract of 1,000 revolvers were numbered for the five companies of US Dragoons, A, B, C, D, and E. Each company letter was followed by the number 1-200. After these 1,000 revolvers there were 100 civilian models made, numbered 1001-1100.
When the 2nd Gen's were introduced, numbering started at 1200. I'm not sure where the 3nd Gen's started but I would guess where the 2nd Gen's left off.
Walker copies have been around since Sammy was still alive, and modern reproductions began in earnest circa the late 1950's-early 1960's.