Yep... Buster chip was my thought, too. Though the ESCOM boards *SHOULD* have been Buster 11's. (Of course, when Escom started running out of parts, who knows what parts they used!!)
Check out Warren Block's
Amiga 4000 hardware information page. It's really an excellent source for all your A4000 questions! It's saved my arse numerous times.
I believe he also has a "definitive Buster" section dealing with Zorro 3 issues.
Here it is...
