The numbers mean one has the calibre 15 (small running seconds) and the other has the calibre 11/12 (hour counter)

These movements are not Valjoux they are the automatic chronograph microrotor movement jointly developed by Heuer, Breitling, Buren et al.

There is some conflicting information on the master reference tables:

The Monza 150.501 and the 110.501 are mixed up.
the Economy version is the one with the cal.15
[110.501] and the other version has a cal.12 - [150.501] these, as can be seen from various catalogues of the era, were more expensive than their Carrera equivalents!

For all monza's the xxx.501 indicates 'black chrome' case, xxx.511 indicates chrome plate case

Here's a picture of mine... a 150.501: