PPG Glass Question

[Dashman]

My Dad's '72 K10 from the St. Louis factory has most LOF glass as such:

Driver's side glass, YN7: October, 1971, Ottawa, IL
Driver's vent glass, XY7: February 1972, Ottawa, IL
Passenger's vent glass, LY7: March, 1972, Ottawa, IL
Rear glass, TN7: August 1971, Ottawa, IL

However, there is this one rogue side glass:
Passenger's side glass, PPG 2659: 265th day of 1969, or 1979?
M27.7: Tinted side glass, 7/32" thick.
26: Crestline, OH. 59: May 1969 or May 1979?

It seems obvious that this is a replacement window. Is it OEM from a 1969 truck, or is it aftermarket from 1979? Why would the "26" and "59" be without a hyphen or space?

Thanks for your responses!