That is what the OP (and some others in this thread, I think, including me) are assuming. That’s what he’s talking about in his fourth paragraph.
Yeah, something like that is the general thrust of what I’m thinking it may be.
48 bytes suggests that it’s the size of a sector on the disk. Alone, it’s practically useless.
What filesystem yields 48-byte sectors? Even a 360 KB floppy disk was usually formatted to FAT-12 with 512-byte sectors, 2 per cluster.
If the OP had posted his or her huge file, I bet someone here would have decoded it by now.
Hmmm… Windows 3 (pretty much the first popular version) didn’t appear IIRC until 1990. The 486 only came out in 1989 and it was f’n expensive at the time. Before that, a computer without a mouse was pretty much a given, I think. Anti-virus, movie maker, etc. - way too recent. You would be lucky to have arcnet in 1989. If it’s not a CHKDSK remnant, odds are it’s something created by an obscure program using their own (non-standard) extensions.
Also note that (IIRC) floppies originally had a limit of 255 (or was it 512?) files per disk/directory, so the creation of multiple tiny files was not a usual tactic.
Do you think the name stood for “MOstly USELESs”?
There was a theory advanced in this thread that it could be a WordPerfect checkpoint file. ftg might tell us what those look like.
ETA this is a good example of why you should store your important documents in some human-readable format if you want to be absolutely sure they will be readable when you need them a few decades from now.
ETA2: some people and organisations seem to be using PDF/A. We’ll have to wait and see how it all goes.
I think everything back then were 8 bit so 0-255 is all it can handle.
In fact, with expanding and cheaper storage options, perhaps the opposite - more issues a bit different from current OP - “I created the file, what the heck for?” Someday I’ll have to dig through what I have stored here, there and everywhere - “Copy of GRUMPY” which was the hard drive contents from my 486 machine… Do I still have the memo my boss wrote in 1993 which ended up on the disc of the computer they basically trashed two years later as thoroughly obsolete?
Instead of lost data we’ll be drowning in obsolete and uncatalogued data.
As an example, every so often I run WINDIRSTAT on a client’s computer to find “why do you still have a snapshot of the accounting database from 2008?” and invariably the answer is, it was a safety backup at the time and forgotten, should have been deleted - but in the era of terabyte drives, a hundred megabytes is forgotten and copied across during wholesale migrations over the years.
I am utterly astonished to report that the answer has finally turned up. Beta Archive, which I’ve never heard of before, reports here (https://www.betaarchive.com/wiki/index.php?title=Microsoft_KB_Archive/50534) that:
If the Word Version 5.00 tutorial is accessed from within Word through the Help Tutorial menu, user lessons are not stored in the tutorial lesson record files, KEYLES.CHK and MOUSELES.CHK. To record individual progress through the tutorial lessons, start the tutorial from outside the Word program by typing "learn" (without the quotation marks).
Which to my reading, says that this file stores progress on the Word 5.0 tutorial. What happened to my copy of KEYLES.CHK I can’t say, but I’m satisfied now that I have an answer as to what this file was back in the day. Word 5.0 for DOS was released in 1989, so it would have been pretty cutting-edge for my computer lab to have it, but I’m not completely shocked.
OK, how’s this for weird?
I have not thought about the Straight Dope since the San Francisco Bay Guardian stopped publishing about a dozen years ago. And I had never heard about this board.
I stumbled on it because I was trying to remember something about Mad Magazine. I have no idea how the two things are connected.
After cruising, I was going to leave the site because everything posted was so old then I saw something had been posted 44 minutes earlier.
And what do you know, in 2018 you asked a question I actually could’ve answered!
In 1989 I was working my way through law school doing computer consulting. I had a client who decided to migrate from WordPerfect to Word 5.0. Yada yada, I had to figure out how the tutorial worked inside the application and directly from DOS.
You did indeed find the correct answer. It’s not surprising people got confused by the .CHK extension since CHKDSK was something we used all the time. in case you need to do some work, CHKDSK /F will actually repair things we’re running it naked will just let you know what the problem are. I’m sure that piece of information will come in extremely handy in 2026.
But it’s extremely unlikely anyone would ever change one of its output files to have the name you discovered.
At that point in computing, most people didn’t have a mouse attached to DOS computers. MICROSOFT had introduced a graphical overlay which could be used with a driver. But it still needed to be navigated with a keyboard and so the tutorial could be run showing how to use command keys.
I could go on, but I won’t except to let you know that while the CHK extension was for checkLIST when used with the disk application with the tutorial it was for checkMARK for reasons you have already figured out.
FYI, I created an account on this fossilized message board just so I could share this useless information.
Welcome to the Dope! And actually you’ll find it’s really quite active, especially for a message board in 2026.
WAG, you started (even by inadvertently clicking on it), the MOUSELES.CHK tutorial, thus creating the progress file, but never started the KEYLES.CHK tutorial.
Welcome to the Dope indeed! Always glad to have more folks here fighting ignorance, even if lately almost all of my posts have been revisiting unanswered questions I asked long ago. 
Joey_P, that’s as good a WAG as any I could have come up with. Again, I’m finally glad to just be able to cross that file off my “well, what the hell is this” list.
Which probably makes you just the sort of person who’d fit in well here. Welcome, and please consider sticking around.