I have posted queries about the origins of these cars on this list 2-3 time hoping one of the USRA experts might respond since given the 5/5/5 Murphy ends, fishbelly underframe, and Andrews trucks I believe their origins derive from the USRA 40-ton DS boxcar. I speculate that FGE got their hands on a group of the USRA cars at a good price and rebuilt them into reefers.
There were 140 cars in this group. I have a FGE re-builders company photo (8-1937 Indiana Harbor) and an in-service photo is in the Larry Kline-Ted Culotta NMRA post war freight car fleet book. The cars have a Hutchins or Hutchins type roof and the steel sheathing consists of three riveted horizontal panels on each side of the doors. A similar sheathing arrangement was used on 100 ex-PRR R7 reefers rebuilt by FGE about the same time. Here is a link to a photo of one of these:
FGE used horizontal sheathing on the 55 new cars they built before WWII, fifty of which carried FDEX reporting marks designating they had FGE's permanent collapsible decking system.
