My 67 doesn't have a bumper, yet. I have been looking and I do know of the slit you are talking about. All the 67 bumpers I have seen have the slit all the way across. On the 66, however, the slit does stop in the license plate area, doesn't go all the way across. Since 67 bumpers are hard to find, perhaps you saw some 67's with 66 bumpers on them.
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Pat Kelley
66 El Camino, daily driver
67 El Camino, STRIP/street