Monterrey currently has a signed grid (http://www.nl.gob.mx/servicios/sistema-de-senalamiento-alfanumerica).
But I found a sign for an apparent former system using pentagonal shields (https://www.google.com/maps/@25.6848992,-100.2713228,3a,43.7y,62.54h,110.34t/data=!3m7!1e1!3m5!1slRLUDzmNNvsWPeLvjYsCiA!2e0!5s20170901T000000!7i13312!8i6656). Does anyone know more about these?
Mexican county routes!
Found another 77 (https://www.google.com/maps/@25.7336343,-100.3154231,3a,19y,230.85h,104.25t/data=!3m6!1e1!3m4!1snXFQiLppYcc_L4hSbNbewA!2e0!7i13312!8i6656). Both 77s are on what's now 210.
I have now spent a lot of time googling this, and I'm coming up short.