Digging through more pictures I found another confirmation of the wierdo 'third frame' - on 2590, which makes zero sense. 2590 built Dec '72 when 2616 was May '72, both had this oddball feature and the units in the middle didn't. After this I 'think' they were all closed frames.
http://www.rrpicturearchives.net/showPicture.aspx?id=50877This appears to the the last unit with an open frame...maybe.
Meanwhile back on the front, I finished up my own 2615, as-built June of '72.
This is the unit that is featured in the kit instructions. This body shell is shown on the modified GP15-1 chassis. Other than the air horn, it's a 'stock' kit build. There's wierd stuff in early '72 that didn't last, like black trucks, yellow stepwells, and the ACI label.




I am going to make a master for the very early cab with the F-unit windows, I enough order confirmations now to go ahead with that.
I'm shipping daily off of the original email list now.