It is definitely an M1 Piano.
That same piano is also featured on the Wavestation EX and Waaavestation A/D as well as M1R and M3R sound modules.
You can also find that same piano on some of the Card Sets for the
01/W series ("Best of M&T" I believe). That sound is also available on a lot of sample CD ROMs.
Even though it is a very old sample, it's kind of a classic and a lot of people still like it (my self included).


-ED-