Thanks, Nick
I was just reading from a book that arrived in the mail a few minutes ago. It's called Cavalcade of Toys, published in 1942. In it there's an etching from an 1875 Union Mfg catalog that shows a workshop very similar to yours. On significant difference is that it's powered by your Buckman horizontal (which oddly reminds me of the Sphynx).
On page 200 they write "A boy with a steam engine needed something to operate with it, and so the miniature machine shop became popular. The Union Manufacturing Company of 13 Vine St., Brooklyn, N. Y. catered to this specialty."
They go on to write, sadly without references, that "The Union Company appears to have taken over in 1880 the 'Young American' engine."
Would you mind if I used pictures from your Flikr album in my article?