Hi Richard,
Yes, its a strange one this.
I started with a Canon A80 but at the time there was no Inon Adaptor for it. I therefore modified the AD DC700 adaptor which was available for a Canon A70. All this actually involved was enlarging the mounting hole on the bottom by filing away some of the metal. The attatched picture shows the enlarged hole. It worked great.
When I upgraded to the A95, again no adaptor was available to start with but I could still get my previously modified adaptor to fit and the results were just fine. When Inon then released a specific adaptor (the AD DC900 for the A80 and A95) I bought it straight away. It was terrible - awfull vignetting on both wide angle and macro lenses. I sent it back for a refund.
My modified adaptor on the other hand solved the vignetting problems by holding the lenses about 4mm closer to the housing front port. I was amazed that the Inon design seemed so far off the mark. Sometimes I wondered if Inon had badged the wrong adaptor for the wrong camera. I am very confident that the optical quality of my set up was not compromised as I've had such good results.
I would therefore recommend testing these adaptors out in a shop somewhere to see if everything fits together. I think there are just too many camera/housing/adaptor combinations for the dealers to understand what they are selling.
Trev