Prior to WWII Colt didn't use letter codes for most of their revolvers.
It was only after WWII that Colt began using the letter codes such as "D" frame for their small frame models.
In the pre-war era frames were usually known by the name of the first model to use that frame. As example, the Detective Special frame was called the Police Positive Special frame, because that was the first model to have that frame.
Following that, the Pocket Positive would probably have been known as the New Pocket frame, since the first model in that size was the New pocket model of 1895.
In the following few years Colt upgraded it to the New Police model, then to the Pocket Positive, then to the Police Positive.
There was a Second Issue Pocket Positive, but I don't think the grip frame changed.
There's more information on the Pocket Positive history on the Colt Fever site....