Actually they didn't even start numbering the books until The Final Reflection came out with a "16" on the cover -- but the backlist inside still had no numbers assigned for it or any of the previous 15 books.
I realise that, but the then-current books (and new reprints) on the shelves
were numbered. The whole marketing trip re numbering attempts to pull in the collectors who are used to hunting for their numbered bubble gum cards. (Scholastic's "Goosebumps" and "The Baby-Sitters' Club" used this feature to huge effect in the 80s.) People bought a few numbered books, realised they were missing some, and often ordered by mail the titles they needed. I understand Pocket had a very healthy mail order trade going until Amazon came along.
So when ST IV came out, Pocket had only been using book numbering for two and a half years. Thus, leaving a book unnumbered wouldn't have been as big a deal as it might seem in retrospect. (Particularly since Enterprise: The First Adventure came out just three months before the ST IV novelization.)
Well, I spoke to a book wholesaler around that time. Pocket had been
amazed that the novelization for ST:TMP, retro numbers as #1, was still in print, due to consumer demand, all those years later. It holds some kind of a record for longevity in-print, IIRC. Also, S&S often sold the international rights for the novelizations to
other publishers (variously Futura, Orbit, Granada, etc, in UK), and they wouldn't be keeping Pocket numbering. After Titan UK let go of the ST line, S&S started to export the numbered books to UK.
I'm thinking there must have been a mass reissue at some point, because there seem to be quite a few books from this era that have 2 different covers.
Yep. When the gold embossed title covers were current, Pocket started
omitting the metallic foil and the embossing
from the second printing. I know with "Deep Domain", the unembossed second printing reached Australia (by air freight) before the sea freighted first editions! I really had to scramble to keep my first editions in order! Pocket started blurring the line on what was a second printing vs second edition.