Is an EASA STC valid if it mentions issue 1 of a Master Drawing List that was later revised?
This sounds like a case where I would contact the STC holder and ask them what's up.
In the USA I'm used to seeing STCs with the magic words or later approved revisions (which means that the manufacturer can revise the document and get it re-approved without needing a new STC), but if a specific revision is called out and those magic words aren't present then later revisions are no longer "approved data" (or whatever the EASA equivalent term is) and technically can't be used.

In all likelihood the two revisions are materially identical and this is just a "paperwork problem", but you're better off being sure (and either getting a STC with the "later approved revisions" wording or getting the original issue of the drawing list so you have it in hand and comply with the STC you have) rather than having your local regulatory authority decide your installation is illegal because you didn't use approved data.  


  If there is a changelog on the document in question your mechanic could theoretically make the determination themselves - Added drawing #123 has no material effect if your installation doesn't reference that drawing. At that point it depends on how picky the folks approving your installation are though - again, they could say since it's a later revision it's not the approved revision.