There are 2 Pulleys listed for a Mk3 Golf.
021 903 119 E - 15
030903119 C - 4-6,10,11
The remarks, in italics, don't mean anything to me but they might mean something to you.
The problem is that alternators recently aren't serviceable parts any more, so you cant really buy parts for them. Best bet is to get both and compare, unless you can find somebody to measure for you