That is, a I or V followed by an M in the superscript.
This might simply be a not-so-good way to indicate that the chord is Major. I usually see this in some kind of off-beat music sheets, usually written by people that don't really know that the capital I indicates that the chord is major or just don't know how to notate the chord numerals.
I disagree that the M means means major 7th (as in Imaj7), because you can deduce that from the 4 2, which means that the chord has a seventh and is in the third inversion.
What is really common, and you might often see, is the lower case m, which means minor, as in Cm or C minor (or Im / im). People extend this writing as C M or C Major (or IM . I Major), but it's not as common. You might see it on some transcriptions of songs on the internet for instance. So amateur musicians might often blend these two together to create a chord name / Roman numeral hybrid.
In my experience, the most common usage is that this "M" signifies that this is a major seventh chord. (But note, as phoog states, that "maj" is the more common usage.)
This is necessary because something like a V7 assumes a minor seventh above the chordal root. As such, we have to clarify that this chord will be a major seventh quality.
Regarding diatonic seventh chords in a major key, this M will only be necessary above the I and IV chords. All other seventh chords either:
assume the minor seventh above the root (ii7, iii7, V7, vi7),
or explicitly tell you the quality of the seventh chord (like for vii°7).