Has anyone built one? Stuart, Reeves, Brunell, and Myers Engine (USA) all offer castings, but I've really only seen photos of the Stuart. I'd love to see pictures of some of the others if someone has built one. Particularly the Reeves offering. The drawing in their catalog is so rough it's hard to tell what it might look like.
I watched a Stuart hammer sell on eBay the other day for £385 - ouch!
However, I have failed to find any pictures of the other makes you refer to. I have found an article in ME 2668 which has a picture of a Nasmyth steam hammer on the front cover (it was in the science museum), and an article on a Massey steam hammer model inside. These are both very similar types with two 'legs', and the hammer inbetween - no drawings though. Steam hammers have appeared from time to time in ME, but this is the only one I have a copy of. I expect one of them will have had some plans.
It is possible one of these models has provided the prototype for the ones you see for sale - but I think they are all single 'leg', like the Stuart - which are presumably for lighter work. Peanuts, instead of walnuts.