Not sure when MD 51 was 1st used but I know it was still in use in 1985.

This came out of the Burt records on chips I have. TLS inlayed chips.
Delivered to Caro & Fils, Chicago distributor in 1985-9,000 plus chips 5¢ to $500.

I believe MD 50 was used much later than 1960.
I have found records in the Portland Card Company records made after 1960.
I did not search my chips but guessing 1970's maybe 80's.