Hey anon, saw that few breds back as well, and yet, while new to you, it's been covered a couple times.
There is also the Gospel of M, or M Source:
(wiki): M Source, which is sometimes referred to as M document, or simply M, comes from the M in "Matthean material". It is a hypothetical textual source for the Gospel of Matthew. M Source is defined as that 'special material' of the Gospel of Matthew that is neither Q source nor Mark.
There is moar on Q Source and the relationship here as well:
https://en.wikipedia.org/wiki/M_Source
Carry on anon..