Confirmation:
I was hunting for a local store that had E12 motors in stock and finally found one, so I drove over there yesterday afternoon and bought a pack of E12-4 motors with the black plugs.
I compared them to a pack of E9-4 and another pack of E12-4 motors I already had.
Here are the observations:
E9-4 date code A 06 22 12:
Black plug fit tight. I used a Starter to measure the depth of the centerbore from the aft face of the nozzle in comparison to the other motors. This one was "not so deep".
E12-4 date code A 06 22 12:
Came with yellow plug. Yellow plug fell right out confirming that they "never fit". Black plug fit OK, but not as snug as the E9. Depth in comparison to the E9 was "deeper".
E12-4 date code A 07 25 16:
Came with black plugs. Black plug fit OK, but not as snug as the E9. Centerbore was deeper than the other E12 from a few years ago and much deeper than the E9. This could simply be a result of more nozzle and the actual propellant centerbore may be identical to the earlier E12 motors.
In other words, there may be no performance change. If someone has a test stand, feel free to test some old and new motors and confirm. I'll assume they are identical or very close to identical unless test results show otherwise.
The Yellow Plug NEVER fit the E12 motor. I've tried since they came out to get the plugged changed. When the person in charge of motors retired I was able to get the plugged changed to one that actually stays in the nozzle.
John Boren