Palitoy Fist Fighter back review courtesy of Tom (who also provided much of the content).
Only Palitoy Fist Fighters have black buttons. When the US FF's were first released. Robin's tunic was obviously modified to allow access to the fist fighting button. This was the only change for Robin. The Batman, Joker, and Riddler FF outfits originally contained two sets of snaps. The first was located at the top of the neck like the regular outfits, while a second set of snaps was located right above the fistfighting button. The additional set of snaps was then dropped as later FF outfits came with only one set of snaps. Mego most likely decided to use standard type 2 outfits on later FF's. Outfits containing one, or two, sets of snaps are accurate.
The slits on the original FF outfits were the same length as slits found on some standard type 1 body outfits. The slit length does vary among outfits of the same character. The slit on type 2 outfits did get smaller when compared to the type 1 outfits. I am not sure if Mego purposely increased the slit length for the FF's or it was just variance among production runs. If the original two snap outfits have the same slit length as some standard T1 outfits, I would guess no specific variation in slit length was intended.
Joker's jacket was initially made with a higher slit in the coat tails to facilitate access to the fist-fighting button. This was specific to the FF Joker, but it is possible that some FF Jokers have the regular jacket and even some regular type 2 Jokers have the jacket containing the higher slit. FF Jokers containing both versions of the jacket are accurate.
When the four additional characters for the Palitoy FF's came out, the outfits for Lizard , Spiderman, Superman, and Captain America were all uniquely modified to facilitate access to the fist-fighting button. These modifications were done with greater detail when compared to the US FF's. Note the underarm slits on the Spiderman pictured above courtesy Steve Caballero.
Note the sewn-on sleeves in this Captain America picture. Superman also had sewn-on sleeves and both have belts that do not go completely around the figure, but instead terminate at the hips. Cap in particular seems to be rather, erm, exposed.
UK Fist Fighter Captain America and Superman details provided by Rob Chatlin.