The main difference between the First Model and the Second Model is the First Model had the Triple Lock feature, and the heavy lug under the barrel. The Second Model did away with the third latch, so the ejector rod only latches at the front and the rear, and did away with the heavy lug under the barrel, leaving just what you see in the photo of the Second Model. The Third Model brought back the heavy underlug, but the Triple Lock feature was gone. The ejector rod latched in place only at the front and rear. The only revolver that Smith ever made that latched the ejector rod at the front AND the rear AND the middle (at the front of the frame) was the First Model 44 Hand Ejector, also known as the Triple Lock. There seems to be some overlap of your Serial Number with First Model and Second Model Serial Numbers.