And here I am answering my own questions again
I just had a rumage through my folders and found the dia compe 86 brochure that someone posted a link up to a while back and both types appear in there (piccy below). For some reason i had it in my head that the 2-piece came out first and was replaced by the hinged in the early eighties but i guess i was wrong
Well it's for an '84 build so i think it'll work just fine
Piccy from the '86 catalogue: