I quite like this kind of problem. Of course investigate Sublime Text as mentioned above, looks as it that might do it.
Otherwise perhaps try emacs. I have no idea of emacs will still support such old stuff.
Here are the notes I made when looking at this.
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PerfectWriter MSS
https://en.wikipedia.org/wiki/Perfect_Writer
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MSS File Extension - Open .MSS File (Manuscript text file)
.MSS extension are known as Manuscript text files
.MSS in reference to Perfect Writer (DOS/Cpm) word processor files, are in text ASCII format, with text used for formatting and it is felt printer controls (Epson compatible for the day)
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https://www.webopedia.com/reference/...le-extensions/
.mss Manuscript text file (Perfect Writer – Scribble – MINCE – Jove)
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https://opensource.apple.com/source/...WS.4.auto.html
Copyright (C) 1992
Changes in version 18.58.
** Scribe mode now exists.
This mode does something useful for editing files of Scribe input.
It is used automatically for files with names ending in ".mss".
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https://en.wikipedia.org/wiki/MINCE
MINCE stands for "MINCE Is Not Complete Emacs"
MINCE was a companion product to SCRIBBLE, a text formatter based on Scribe
MINCE and SCRIBBLE were later developed into the Perfect Writer and FinalWord word processors. FinalWord later became Sprint.