Should I fit 2nd hand servo?

[spaarky]

Longstanding engine problem finally traced to air leak in brake servo - looked at online VAG catalog which suggested new servo was 314euros ! So went looking for breakers & got 2nd hand servo at a v good price, but am confused as to whether it will fit/work since part numbers all different ! Anyone come across this before - should I be worried or just fit it & hope ?

Broken servo on car has part number 894 612 103A (Girling)
VAG catalog says part no should be 893 612 107A (Girling)
Replacement from breakers has no 893 612 105E (Girling)

Car is 1988/89 B3 80 quattro with ABS, but is my only daily driver, so am nervy of taking it all apart & then finding the replacement servo won't fit, leaving me off the road. Am bit confused since I thought all B3 used the same servo ?? Or is it just that audi played with their part no.s quite a lot & I'm worrying too much ?