snare drum age

by John
(Cambridge, Ontairo, Canada)

I have recently aquired a Ludwig Supraphonic, 6-1/2 x 14", Chrome, 10 lug snare drum. The serial number is 7 digits - "1846135". I believe it is from the late 1970s. Either 1978 or 1979 when Ludwig started using 7 digit serial numbers. Can someone confirm the actual year? Is it possible to zero in on the month?

Thank you
John