Have found the 2-volume Argentina specialized catalogue available from Jalil stamps in Argentina (and written by them) a very useful reference when trying to sort out classic Argentina. Happy to provide additional info to anyone.
If you're getting in depth into Argentina collecting, I highly recommend it. Well illustrated, covers everything you'd expect and more - varieties, watermarks (as we saw!), officials, postal stationery, prephilatelic cancels, Corrientes, etc... In Spanish.
Ordered last year, cost was around $100 USD delivered to Canada.
https://jalilstamps.com.