The switcher you are looking for is actually an "MP15DC". It differs from the MP15AC in that the DC version has a DC main generator, while the AC version has an AC main "alternator". The AC and DC versions are also very different on the outside. The easy way to tell the AC version is that it has see-through radiator intake grills low on the sides, instead of in front, akin to a SD45T-2/SD40T-2 tunnel motor.
The traction motors; however, on both versions were still DC motors, not to be confused with today's AC traction motor equipped locos, like the SD70ACe. EMD was using the DC/AC suffix on their model designations at that time since it was a transition period when AC alternators were starting to appear as an option on their locomotives that were once produced with either DC main generators or AC alternators. They also used this suffix on their GP38AC units.
Overland may have produced an SP detailed MP15AC in brass but they are going to be more rare than the Atlas MP15DC version, that's for sure 
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Here is a link to more detailed info on the MP15DC.
http://en.wikipedia.org/wiki/EMD_MP15DC
Also, in this issue of OST there is a great article on weathering the Atlas SP MP15DC and it is free to download.
http://www.oscalemag.com/docs/ost_44.pdf
Good luck in your search. The SP version is definitely a looker in my book.
Scott K.
Austin, TX
