69NV,
That looks like information you gathered from Pacecar Jeff's website, AKA
www.broachbuster.com.
I think it is fairly accurate and I do know that Jeff has spent a fair amount of time reseaching this stuff. I think his radiator hoses in particular are very nice. The original alternator on my 66 L79 Nova did have an "A" by the plug and an "NL" on the case though it was very faint and almost unreadable, but that was almost 30 years ago when the car was only 18 years old. I'm not sure what he means by "A" being on all 66 thru 75 alternators, and then he says an "H" is on all 66 thru 75 alternators??
I think you can rely on his findings, but, if you find out differently down the road a little lacquer thinner on a rag will take care of it real quick and then you can apply the new code.
BTW, If you have the build sheet for the car the 2 letter code can be found on it.
Dave