You are correct in the fact that I missed 60, but I was earlier corrected my Doug who stated: Doug’s information is more accurate.
I just as soon not have to have a formula to figure out the grade from one scale to the next. Had enough Celsius, Fahrenheit, and Kelvin to last 2 lifetimes.
That's because you're only looking at NGC and PCGS. But before either one of them ever existed there was another game in town - ANACS. And in 1977 there were only THREE MS grades - 60, 65, and 70. And ANACS graded many coins as a 70.
I'd give those as asterisk, I can't consider it grading with only three grades everything is getting rounded too
I've never seen an early ANACS certificate that graded anything MS-70. I would be surprised if ANACS graded anything as a 70 until sometime in the 1990's, possibly even later.
You merely making a personal choice. At the time, those WERE the only MS grades that existed. And those graded as 70 were the first coins graded 70. My response is an accurate answer to the question that was asked.