Not rocket science. Three mirrors used. Some early years show Day/Night STD later years optional.
Without a window sticker or pic from day one, pick which one you like.
Base 10 inch and 10 inch day night (Prismatic) and 12 inch Prism
Mostly higher end premiums got the big 12 inch Prismatic. And then there are the ones who just wanted a day/night or bigger mirror. 2 different types of base none adjustable used one like in my pic and the other a full flat back.
Anything can happen over 40 plus years. How many in a junkyard might of said... hmm this is a nicer mirror then mine?
ASP500 What does your 79 parts book show on mirrors, can you post pic?
The parts book uses tricky language.