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.
The characters you were typing are (probably) the code for a jump to an entry point somewhere else in the file.
I think what parent is remembering were characters that effectively created a jump instruction at the beginning of the file.
[1] https://en.wikipedia.org/wiki/DOS_MZ_executable
edit: [2] Section 6 of the paper talks about that, just noticed.