OEM Ignition/Door keys

[eric8]

Being one to obsess over genuine OEM details when it comes to my 77 C10, my next quest is to find the proper NOS ignition and door key. All I have are the Ace Hardware versions.
Based on my research, I need an E and H coded key for the ignition and door (GM# 1154606 & 1154607, respectively). Looking at GM parts catalogs, it seems GM changed these key codes every 4 years? For instance, E and H blanks are for 69, 73, 77, and 81 model years.
This of course piqued my curiosity. Even though every GM key blank looks the same, I assume there are subtle differences that won’t let a 1976 key blank be cut to fit a 1977 ignition?
Can anyone shed light how this works? Yes it’s mainly me nerding out for my own curiosity.