Looks like a 20 whit a later (or replacement as the originals failed) crown black plastic dials were replaced by metal ones with sharp corners on the 21's
IMHO the brace is the original one but cut and repainted, the canti hanger is missing. I´ve done this couple of times myself
Not sure about the 20/21 issue, I´d say Mag 20.
MAG20 (1992) main visual difference is the lower legs are different style, different crown er different brace
Visual element that make it a 1993MAG21 (and not 1994/5/6+):
Brace : has three holes for the canti stud, you cannot really see the other difference in the pictures (you can see the chop marks on the brace in the bottom picture by the way)
Crown : Classic ring under the crown where it clamps a stanchion being the easiest.
TopCap: 1993 style, 1994 they moved to larger machined get your finger in style.
Of course all these can be jiggled with other years as can most of the internals