Update: I have my data well sorted enough that I found it :-) It even comes with code and is under a vague free license: https://github.com/hannob/com2txt
* com2txt.exe is 7110 bytes long, xcom.com is only 401 bytes long.
* xcom.com can also convert back from text to binary.
* xcom.com can also convert to data text (without the self-decoder header).
Kind of uncanny to see a perfectly readable Makefile written around the year I was born...
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.
The characters you were typing are (probably) the code for a jump to an entry point somewhere else in the file.
First two paragraphs of the README:
Com2txt is a tool on MS-DOS which converts a com file to a text file. It's DOS generic. Unlike tools such as uuencode, the text file generated by com2txt works as a com file, exactly like the original com file does. Using com2txt, you can create a com file which can be sent through networks such as internet, and runs without any decoding.
Moreover, the text file got by com2txt consists only of ECHOable characters; it doesn't contain characters such as `<' or `|'. So, using ECHO command, you can easily generate the textized com file and use it in a batch file. For detail see section 4.
Crazy fun stuff though.