Brian,

I suspect the situation with early Marriage Certificates is the same as with Birth Certificates: the Registry have in the past, and perhaps still do, send out something that looks like a Certified Copy of an Entry in a Register of Births, i.e. a Birth Certificate, but which is actually a copy of an entry in the (certified copy of) Parish Baptismal registers.

I found this out to my cost when I requested a Birth Certificate before I knew that the start of Manx Civil Registration was quite a bit after the English one. In my case, the parish register didn't add anything useful to the IGI, such as mother's maiden name, and, after feeling cross at the wasted expense and suspense, thought it might have been good customer service for the Registry to advise applicants that they are asking for something that doesn't exist before supplying a poor substitute at the same cost!