What Liza Gervais says is true.
I would like to add that M. may not stand for Marie. You are assuming something. In the old baptismal records written in French, Marie is often missing. In modern baptismal certificates, it's supposed to be always in there, unless a hyphenated first name includes it.
As Liza explains, M. could stand for Marie, but it could also be another first name altogether, although Marie is the most likely. I would put M. in the additional field for given names. If you or somebody else ever finds out what the full name is, this is when you can add it in the preferred first name field. Obviously, she must have gone by Apoline most of the time, but the odd record shows that she had at least one additional given name that was used and known by her contemporaries. It starts with M., and that's all you know until you find more records, like her baptismal certificate, the baptismal record in the church book, or something else.
