Welcome to the forum, Timbo.
I've got a few D21 manuals and one supplemental manual covers the 1992 model. It's 33Mb in size, not too sure where I got it from but if your email provider doesn't mind a file that big, I can send it to you. It does have some electrical information in it, but you might also want the previous manual (1990 original manual, 140Mb).