While the 645 was S&W's first attempt at a .45cal semiautomatic pistol, it was a 2nd Gen pistol and not a 1st. I suppose it is classified as a 2nd Gem because of WHEN it was manufactured and marketed and also due to it's 3-digit model name, but it is worth noting that the two 2nd Gen .45's (645/745) are the only 2nd Gens in the line to have the pressed-in (non-removable) barrel bushing that became the norm in the 3rd Gens.

The only 1st Gens are the 39's, 59's and 52's, and the scarce rare ones like pre-39, 147A, 44, etc.