You are interpreting it correctly as a one of one. That said, it did take 11 lines to get down to a one of one designation. It is often said that each car is a one of one if you consider the VIN.
If your Marti report does not identify the power steering option, then it was added outside of the factory. The good news is that it sounds like it is era correct. Without dealer paperwork, it is impossible to know who did it and when. Dealer paperwork is very rare, it would have to been saved by the original owner and passed on through every purchase to you. Highly unlikely IMO.
How does it affect value? That depends on a lot of variables. When you go to sell, it might just be to someone that is on the cusp of needing shoulder replacement, and they might not want to buy a car with armstrong steering. Then again they might not want a 4-speed either! This is one of those "it's all about choices" situations.