Considering the number of actual panels used in rail cars (not so many as could be made in one tool) I guess there were only a few (or maybe one) die.
Besides the USRA cars they were used in NYC's box cars & 40 ft. gons. 7/8 Murphy box car ends were pretty popular with other railroads too. Maybe the die was originally 7 ribs.
We actually have a single "Murphy" panel (not sure how many ribs it has) - IM's USRA gon.
If you compare the IM end with an F&C NYC gon end you'll find the ribs are longer on the IM end.
