It looks quite similar to the PM Research all bronze early version of their #1 Steam Drilling Engine. However, there appears to be some differences such that I'm not willing to say that is definitely what it is. Rather close though, so it is at least a similar model. The PMR engine has a 1 1/2" stroke which you should be able to measure, and it has a 1" bore, which would be somewhat more difficult to measure, but doable.
I'll say that the PMR #1 is possibly a correct I.D., maybe even probably, but just not quite certainly!?!?
Very nice engine, no matter what, actually!