What does the M stand for on the reply to a wedding invitation?

This is from blueorchidblog.com:

When you receive an RSVP card for a wedding, it will more than likely have a line that looks like this:

M_____________________

So what does the M mean, anyway? Quite simply it is there for you to complete your title when you fill in your name. For example:

Ms. Jane Doe

Miss Jane Doe

Mr. and Mrs. John Smith

Mr. John Smith

Mrs. John Smith

This is the traditional method for addressing RSVP cards, but leaves little room for other titles such as Dr. or military designations. Many couples today are opting to omit the M and simply precede the line with the Word 'Name' or 'Names of Guests' instead.
* Mr. or Mrs. or iMiss. or Ms. plus your name follow it.

Mr. John Brown and family (3) is an example of how to fill that in. The (3) indicates the number of persons in your party who will be attending.
* Mr. or Mrs. or iMiss. or Ms. plus your name follow it.

Mr. John Brown and family (3) is an example of how to fill that in. The (3) indicates the number of persons in your party who will be attending.