Then that was a completely odd set of locomotives you received there. Only SD40T-2, SD45, SD7, SD9, GP7 and a couple others were 5000 series road diesels.
The others were Passenger.
Looks to me like your received a D&RGW GP60 but somehow with a dynamic unit from a GP40.
Stuff happens. Besides the fact I cannot find anywhere an Athearn pat number for a D&RGW GP50 unless you still have the box.
How long ago did Athearn introduce its RTR SD50s? If it was three years ago, I think maybe he saw a listing for SD50s and somehow got GP60s.
The dynamic brake housing on the model looks nothing like that on the prototype. It does look like a GP60 dynamic brake housing. My Athearn GP60s have the water sight glass. I stand by my statement that those are GP60s.
GP60s have square dynamic brake housing (except the early ones), GP50s do not. You are right that DRGW never had GP50s. Athearn never made GP50s painted for DRGW.