Tom ,
DAPCO = Detroit Automotive Products Co. The original name of the company. My notes are very few were done by SAI ( mainly by tag codes ).
GT350Lad
Can you post a picture of your tag with the L on it? My notes (again) are that '66s ordered with a locker , had the locker in the trunk . IF it came with it installed , the change would have been done at High Performance Motors (IMHO) before the sale , similar to the way radios were done. I haven't seen a C6ZX tag with the L designation in all my years of owning and studying these cars. There is always room for a first and I am anxious to add to my "I've seen that" experience. Not doubting you at all.
Randy