At $24, in 1985 money, I would hope it was more than a booklet. I have only seen the two booklet versions and the box sets.

Wikipedia user "Patelasia" thinks the 1979 box set got reprinted with a different cover in 1984:

Boot Hill (role-playing game): Difference between revisions - Wikipedia

I purchased some of the black & white copies that were produced at the time, so I am certain that the booklets were produced.  I didn't purchase the Boot Hill B&W version, though, so I can't say it was produced for certain.

blackdougal: Thanks for posting that catalog listing, I hadn't seen that particular one before.

I've never seen the B&W Boot Hill, but I did have a copy of the B&W Blackmoor Supplement reprint. I sold it off on Ebay long ago, and IIRC harami2000/faro bought my copy. These have been mentioned a few times in these forums over the years, and there's a note about them on the Supplements page:

Note: black-and-white reprints of the Nov 79 editions were also later released, probably as stocks of that print ran out.  They are basically photocopies; some sources have reported paper-thin covers, while others describe covers identical in weight to the normal tan-colored versions.

Supplements

I ordered it from a later version of the Hobby Shop Catalog ('88 or '89?) and I don't recall that it was identified like it is here. If I located my later catalogs I will take a look.
Even if it was identified at the time, I didn't notice & didn't realize it was out of the ordinary until years later when I started reading the Acaeum.

I do have a Summer/Fall 1986 Edition catalog handy that I got back in the day that more briefly lists each of the OD&D supplements as "Black & White Booklet" in the "D&D Original Collectors Set" section. But there's no further explanation of what this means, so I don't think I noticed it back then.

I wonder if they ran out of boxes and were simply selling the book and map that were left over?

Also, 1985 was a great year for vapourware.  If you look through the 1985 TSR catalog, it has a number of products that were never produced: Proton Fire RPG, TSR's Conversion System for RPGs, Star Frontiers Battlesystem, Parent's Guide to D&D, etc.
Note: no mention of Boot Hill in this catalog so I maybe lean towards it was a way for TSR to get rid of remaining stock.