Theoretically it *might* work. When I flashed my 920 with different Country Variant firmwares, I noticed a difference in which of the fastest speeds I could select in the dropdown (2G, 3G, 4G). One country's firmware I could only select 2G as the fastest.
So, maybe an AT&T 1520 will be flashable with a RM-938 firmware. You likely won't be able to go back to AT&T's ROM if it doesn't work. I noticed this on my AT&T 920...I couldn't flash the AT&T ROM back after trying others.