I am accustomed to seeing 1960s Slingerland date stamps in the "MON YR" format (March 1966 would be MAR 66). They typically appear as large and thick characters and are stamped in black ink. See the first two pictures for examples of OCT 65 and NOV 66 date stamps.
There is a drum currently on eBay that has a different style of date stamp that one normally associates with Ludwig. It is the "MON ## 19##" format (March 12, 1963 would be MAR 12 1963). The characters are much smaller and not as thick. The third picture is from that listing. It has a NOV 1 1965 date.
I am not very well versed in Slingerland, but I have been casually recording serial numbers and dates stamps on Slingerland drums for about three years and had not seen this during the era. Has anyone else seen Slingerland date stamps in this "month date year" format during the 1960s?