Welcome to the forum. Neat little guns and it is very easy to get confused with some of these old timers. The naming of the early tip ups reminds me of engineers or state planners that number highway exits in order and then years later decide to build an off ramp between two existing exits.
On the MA turnpike, the construction of route 495 required an exit number of 11A since 11 and 12 had already been assigned.
Similarly as others have stated, S&W built the model 1's and 2's so that when they came out with a smaller .32 than the model 2 they had to call it the model 1 1/2.