Shimano Date Codes
 

Recently acquired an Austro-Daimler Inter-10 with early Shimano 600 components. The date codes on the front derailleur and brake calipers are ZL and ZH respectively. What I'm finding on the internet has Shimano date codes beginning in 1976 with "A.." Are these components pre-1976, maybe by one year?

EDIT: The front derailleur is a Shimano 60, but the brakes are Shimano 600, as is the rear derailleur (though I couldn't find a date code - still on the bike)