If you read the paper I think you'll find it is something like "ZM~~_#____PRinty__C", where _ is a space, (HTML compresses spaces). See section 8, also the beginning of the paper.
.COM files were straight binary, no header and loaded at address 256 (0x100) into any 64KB segment. All indirections were local to that segment, hence the 64KB limit of .com files.
The characters you were typing are (probably) the code for a jump to an entry point somewhere else in the file.
No, COM files had no header. [2] I think that's part of why they were replaced. ZM was for EXEs. I think that one was someone's initials. (Yes, looked it up. [1])
I think what parent is remembering were characters that effectively created a jump instruction at the beginning of the file.
