About the SECOND SN digit on SK 1965-1969?

[SlowBowke]

Ok, even a simpliton like me understands the first digit being the last digit of the year made on Grayling Bear bows from 65-69 but I have a question.

Mostly Ive seen the letter, the second digit be a reference to MODEL. I had a 66 K with 6L and a 66 K Mag with 6M and assumed mostly that the letter ran the same per year on each model.
And it also changes per year, as the first digit does?

Ive seen two 64 inch Super Ks.

One was a 67 1/2 with serial number starting with 7Y
The second was a 69 with serial number starting with 9S.

So are ALL 67 1/2 64 inch models going to have a 7Y start? and ALL 69 64 inch models a 9S?

OR...IS THERE some OTHER significance to the letter which is the second digit on these bows?

Sorry if this is common knowledge but I put in several searches and am not finding a thing.

If already discussed and you know the name of the thread, fire it at me? Also a little curious as to what info I find that some DONT like the 64 inch versions. Some even say they don't last as long?? "Stress" crack more? Any downsides here I should know about?

Somewhere, someone has to have a list of this info..(I hope). It isnt really crutial since its obvious what year it was made but it had me wondering about the second digit, the letter and where it came from.

Thanks for replies in advance...once again.

Steve (Slowbowke)