It's an "E & I" frame.
The Colt Official Police and Python guns are typically described as "E & I" frame guns, since early 1950's versions were "E" frames, and the later versions were "I" frames.
Without trying to confuse the issue, pre-war medium framed Colt's were known to the factory as the "41" frame, because it's direct ancestor was a .41 caliber revolver.
It was only AFTER WWII that Colt started assigning letter codes to their frames.
The early post-War medium frame guns were "E" frames.
Later in the 50's VERY slight changes were made and it was renamed the "I" frame.
These are usually just known as the "E & I" frame since they are almost indistinguishable.
The Colt "J" frame was the Colt Trooper Mark III and Lawman.
For grips, buy grips listed as fitting the "E & I", or for the Official Police or Python.
