I was checking the few Mexican stamps I have and was struck
by the print quality of the long definitive series
which was issued between 1959 and the middle seventies.
Not pretty but quite interesting. The lower values were printed
photogravure and the higher ones engraved.
Lots of varieties there including different perfs and watermarks.
I usually find it difficult to spot some watermarks so I tried
the method (more or less) suggested by postmaster.
I believe the first 20c is Scott 878 (wmk 300) and the last Scott 946 (wmk 350 vertical)

Watermark 300 ?

Watermark 350 vertical

From the 50c I only have four and they're all different as you can see.
Probably Scott 881, 949, 1091

I wish I had more of this set for comparison.
Like I said these stamps are not pretty but great fun to sort
through.