The SMLE doesn't have a straight hole back from the barrel to the bolt. It has a smaller-diameter web the bolt head pokes into, sort of like the "extra web" on a 98 Mauser.
The .45-70 rim is too big to go through the hole. It's not critical, and could probably be opened up with a Dremel, but in the interests of overkill I made a fixture to hold the receiver on the lathe. I need to make a stepped bushing to press into the front for the steady rest to ride.
The bolt keeps the receiver from turning on the shaft, and the odd metal wing is a counterweight for to balance the magwell/stock socket. I used bolts and washers to adjust the weight until it balanced.