Whew! Took me awhile, Robert... staring at the inside and outside pictures... for the light to go on, or at least I think it went on... =] The bolt holding the bushing in sets in the bushing hole, short of the turning shaft, and prevents rotation. Like a person would turn the bolt in to light touch on the shaft then back it off to clear. I don't know, something seems odd about that so maybe I'm not getting it after all.
The more I look at your hammer and our hammer, the more I see differences and I'm not sure it's differences in drive-type or size. I think it's difference in time but I've got no way to tell and the only two people I know of who might know are Doug Freund and Bruce Wallace. I've got some drawings of 50# sowblock and dies and there are double numbers in a few places... one set hand written it. Like there are two time versions... at least.
The bandsawing I did building a sow block and set of dies (die's form based on what Sid has found to work well sith LG's, so not true Fairbanks dimensions) was roughing out... cut to 1/16" of a generous line. Still, my friend Joe's (40 year tool and die maker) #1 Bridgeport struggled... wrong tool for the job. "Bear" cuts darn close and grinds to size. He'd use frozen peanut butter for dies if it was cheap and he could figure a way to keep it frozen. Talking with Sid, he felt 1018/1020 was fine for the sow block and he and Keri use 4140 for dies... heat treated to 46-48 RHc. I was going to flame harden the dies Joe and I built but talked myself out of it. Sent them to Sid and Keri and they ran them to their heat treater when they had a load of their own dies to be done (took months... so you know).
If you are looking to build slip-in dies, and they are not complicated, A-36 works fine enough. They will wear out but building new ones isn't a problem if they are simple. The reality is a person runs a hammer on less-than-hot steel. The dies have to be able to live through that so they need to be harder than mild steel is. There are folks using A-36 dies in their hydraulic presses and getting along just fine (hyd. press dies get a lot more heat contact than hammer dies do but the beating isn't there). If a person avoided hammering cold, soft dies would probably work. Sid will have an outlook on the subject... there's no way he hasn't gone down a number of different roads on this already.
A little aside... Jerry Rados (2 hammers, no press, one at 500# and a baby one at 200#) used H-13 for his dies and heat treated them in a cut-in-half water tank full of motor oil (cut as a trough). He made them bainite... stood over the oil (heated around 400F), straddling the quench tank and lowered the red hot dies into it... said he was really hoping the oil wouldn't flash... =] Also said he'll never make bainite dies again... too soft.
Our hammer has a totally different hammer number set up... on "United Hammer Co." plate with other info on the off side (not in pics) and number stamped... one of the differences between yours and ours I was noticing.
Too much yapping...
Mike