Replying to my own comment since I was curious - it was certainly Microsoft MASM, and cc was the Microsoft C Compiler as well. [1] So probably Intel syntax.
There was also something called "SCO UNIX" which was apparently more of a straight adaptation of AT&T System V and might have used a pcc compiler and AT&T syntax. It's hard to figure out what happened when because there are a zillion different versions of Xenix/SCO Unix/OpenServer out there.