Nice desirable item
I have several similar- beam engines. Horizontal twins, compound engines
The pre eminent supplier of kits was a firm called Stuart-turner who sold kits of raw castings kits of machined parts or ( I think) even complete kits
Yours is a model of an engine usually found in ships- triple expansion as, high pressure steam entered the first stage (smallest) cylinder then exhausted having expanded and cooled to drive the middle sized cylinder after which the same process led the steam to the 3rd and final (largest) cylinder
This gave an efficient use of steam, which usually exhausts to atmosphere on the models ( a closed loop condenser/recovery system on ships)
For ease and safety, lots of model engineers run these on compressed air to show them working.
Hope this helps
Pm me if you want any more info
Goat