Thanks, very interesting!
I want to say I’ve seen a date stamp earlier than that somewhere...but that may just be a memory mis-fire on my part!
The oldest date-stamped Diana that I own is in line with your note: a model 60 match rifle with “06 67” on the rear receiver. Another intriguing detail on this rifle, is that the year of manufacture appears to be appended to the front of the serial number, which is 672301x; in other words the “real” serial may just be 2301x.
I have another very late model 60, which follows the same pattern; dated “08 82,” with serial 823692x.