Ian's Cranstone's list of reporting marks show QREX as assigned to:
QuakerCity Refrigerator Line (7/1930-7/1932)
QuakerCity Refrigerator Line (General American Tank Car Corp. 7/1935)
General American Transportation Corp. (7/1937)
I've noticed quite a few models carry the QREX reporting marks. These include the following companies . . .
I only found a prototype photo for Schlosser Brothers Dairy Products.
Can anyone verify prototypes for the other QREX reefers?
See Chapter 9 in Hendrickson and Kaminski's book on Billboard Reefers, which covers Quaker, and includes these and other examples of prototype paint schemes. Full disclosure: it's a Signature Press book.