4k part numbers

["Luke Vinogradov" <lukev@mail.com>]

Please can someone confirm the meaning of the last character in a
VW/Audi part number.  I suspect it's a "series" or "update" identifier
which is backwards compatible with older parts.  Specifically, my '81 4k
fusebox is labelled:

   "171 941 813 B
    171 941 821 B
    811 941 813 (no letter)"

Can I use a part that I've seen on a vendor's website labelled

   "171941821C
    FUSE BOX DASHER DIESEL,SCIROCCO,RABBIT"

I was going to try to crack mine open and fix it, but if the above
number will work (no mods, no splicing) then I'll trash mine and go
brand new.

So is the C just an improved, but compatible, version of the B?  Please
help!

Thanks!

Luke