Since early 1971, this was how Royal did it:
From 1971 to 1976, it went like this:
Example: 17C-H-15940
1 = calendar year built (1971)
7 = decade of calendar year (1970s)
C = month of calendar year (March)
(until 1985, "I" was skipped to avoid confusion with the number "1" and so during that...