AHM was an importer/distributor of entry level model railroad equipment in the 1960's and 1970's. They went out of business and were succeded by IHC. Their stuff was mostly inexpensive. They handled Lima, Rivarossi, Mehano and others. I have some from my younger days and I run it now and then. The S1/S2 refers to the fact it was a model of an ALCo(American Locomotive Co.) model S1 or S2.