You said "an RC baptism on the 10 Dec 1875, with date of birth given of 6th December"

Personally I would go by the church records as they more closely resemble the tradition of the times "baptize them fast". if you are able to get a look at the original church record book you could compare the baptisms surrounding this one for conflicts in dates as a cross reference.

I have seen quite a few registrations off in order to comply with the law of registering the child in the correct period of time before being penalized.