The subject came up on the SAAC site and since I responded and posted pictures thought that the information would be of value in the long run here also.
The markings - PLEXIGLAS
have been found over the years on a few original 66 quarter windows. I noticed them long ago since that is one of the oddities I found on my wife's 66 (shown in the pictures) The marks are normally found in the lower leading corner of the windows so someone took the effort (either with each sheet or with the original pattern used to cut them out)
Believe/recall each sheet of material that the windows were cut from had a single embossed stamp in the corner of the sheet. It appears that the pattern used to cut all the windows allowed (guess they were maximizing the number of windows per sheet) for these to be on the final product rather than cut off as scrape.
This would account for the small number of ones we see with the marking IMHO Have seen very few. Never took the time to see how many could be arranged on a 4x8' sheet of the material. But it the percentage would be few.
Don't know how many examples have been found with stamped ones on each side of the car. In the example of the wife's car someone took the effort to find one of each so they were installed as a match pair. They are formed so there is a drivers and a passenger specific side for each. Maybe just got lucky or the guy was bored and broke up the day by doing this