Found Bob Cutler's TEC website with his excellent article on early American makers. The "Holly" engines from the 1880s were nearly identical to Edgar Side's with exactly the same dimensions, but the maker of the Holly is unknown (Edgar Side?). Any other info greatly appreciated. Nothing on the site on Side except a photo of one of the engines.
Thanks. I'm finally starting to work on part 2 of the series, which is on Buckman/Union Toy.
The wildcard is Ryder. We know that Ryder produced at least one of these over-type engines, as we have an ad that matches the engine that
@Nick owns. There's a picture of his engine in the in the article. Ryder was also selling parts for others to make engines. It's possible he was buying parts as well.
A mismarked Ryder engine like Nick's just sold.