The correct butt plate would be the one with a setter with his right foot on a rock.
If the for-end will not snap on it is a good chance the for-end spring is too short and not catching in the barrel loop.
Not sure what the "E is for on the barrels.
When you get a chance could you send me the whole serial number, length of barrels so I can put them in my records. email@example.com Thanks