There were three major issues of the Model 1. When identifying a Model 1, the first thing that needs to be ascertained is the issue.
If your gun has a round barrel and a fluted cylinder, then it’s a third issue gun. Third issue guns can also be identified by the round shape grip (sometimes called a “birds head grip”).
If your gun has an octagonal barrel, then you’ll need to look at the left side of the frame to identify the issue. Guns with a small circular side plate are 1st issue guns, while guns with a flat irregular-shaped side plate are second issue guns.