Since its the same basic mirror for 65-68 on A,B and F bodies, I believe it's the angle of the mirror head to suit the different bodies that determines the different part numbers.
For example my 68 GTO remote base has the 9787847 part number.
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Ed
1968 GTO (Thanks Mom)
2006 Silverado
2007 Cadillac SRX
2015 Chevy Express 
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