Butyrate bobbin question

Good evening,

anybody knows which year Gibson switched from Butyrate to plastic bobbins?

Thanks in advance

Since no one has answered here, why don't you try emailing Jon at ThroBak pickups? He might know. :hmm

"In 1965 another change took place, the bobbins no longer had the distinctive square hole in a ring as found on earlier versions, but instead had a “T” embossed on top and later, around 1967, the material was also changed from butyrate to ABS. According to some experts, the different bobbins also affected tone, as the ABS bobbins had slightly different inner dimensions, making for a change in winding parameters, thus giving the T-Top models an audibly different tonal character than previous Patent Number pickups, even if the wire used was the same." - The Gibson "P.A.F." Humbucking Pickup: From Myth to Reality (Milan & Finnerty)