IIRC the original I frame had a leaf mainspring (Shown left below) where the improved I frame had a coil mainspring (shown right below without strain screw).


The J frame was introduced with a coil mainspring and had a bigger cylinder window and associated longer cylinder ,

If you focus on the distance from the front of the trigger guard to the edge of the frame in the picture below you will notice is longer on the J frame (top gun) than on the I frame pictured below it:


Terriers and Kit guns were originally built on I frames but changed to being made on J frames at some point.