The Swimaster 3F98 Mini Mark X fin is marked as "new" in the 1979 Voit-Swimaster catalogue:
View attachment 553372
View attachment 553373
My collection of scans of AMF-Voit-Swimaster catalogues is incomplete. I have the 1977 edition, where these fins are conspicuous by their absence, but I don't have the 1978 version or any of the company's post-1979 catalogues, so I cannot say how long the fins remained in production. The fins pictured above in the 1979 catalogue are marked "Made in USA", which indicates that they are the "original" 3F98s. Voit-Swimaster outsourced fin production to Malaysia during the 1980s, so if the fins you purchased are labelled "Made in Malaysia", they will have been manufactured later than 1979.
The name "Swimaster 3F98 Mini Mark X fin" implies that a series of Mark X fins preceded it. The Swimaster 2F93 Mark X fin appeared in 1974 as an unashamed Jet fin clone:
View attachment 553375
The 1977 catalogue expanded the range of vented fins:
View attachment 553376
So your fins represent a new series of Jet fin clones made from gum rubber. This choice of material marks a return to the 1950s, when Duck Feet fins reigned supreme and gum rubber fins were associated with excellent quality and durability. Your fins may be heavier than those made from so-called "technopolymers" or "space-age" materials, but they will be relatively indestructible and their performance will not disappoint you if your kick is sufficiently powerful. I can't speak from personal experience as I only use full-foot fins.
Do let us know how you get on with your recent purchase. A pair of gum-rubber fins for just over 6 bucks is a real bargain.