My understanding is that 'generally' plastic lenses became available during the 1951 model year for many makes of cars, which would be from 1950 calendar year. Due to the odd KF set-up re 49/50 and (ISTR, could be mistaken) the unusually long 51 model year, perhaps the glass are from the 49 production and the plastic lenses are from 1950 production? That glass was ON the 50 cars was because they were actually reserialed 1949 models already built, but if 1950 cars had been produced after a certain date they would have had the same design lenses, but in plastic? As such, the replacements were switched over to plastic, and either stock of leftover glass already shipped in 1948-49 or newer plastic were available. Seems possible to me...