they didn't come with a Gemini diff but the rules allowed for one to be fitted if people wanted to. So some had them and others didn't.
While we're on the subject, if anyone has any spare parts for a gemini diff, I'd be very interested in buying some! Especially the top cover plate.
Ping me an email about the bits you need Hayes, a colleague was one of the owners of Gemini - so might have the drawings somewhere still. I know that the spares got scrapped some time after Ricardo bought them out.