Bill
32055
Seems to be specific to a serial range 1,5xx,xxx for 66 and 1,7xx,xxx for 67
No explation as to why but just where they seem to appear.
15XXXXX, case back dated VI 66 (model 1016)
15XXXXX, case back dated VI 66 (model 6466)
1,7XX,XXX case back dated IV.67 (model 1016)
1,47X,XXX case back dated IV.66 (model 1016)

Rolex 1016 VI.66 caseback 1.5 serial

Both pictures credit Bobwatches