My 1966 San Jose "K" Fastback was built in mid October and it's relatively untouched so its hardware should be close. The picture shows two different types of screws I found on my rear valence. The round washer head screw on the left, documented as 380650-S101 10-12X3/4 in the 66 body assembly manual, pg63, was the prefered screw, qty 11 (I found 9). The wafer head drill tip screws, I found to be 381997-S2 10-16X1/2, were the initial two installed for painting. The were installed in the second hole from each end. As those two screws were installed prior to painting and at a different assembly station, a different type of screw was possible. In the Ford Car Parts book, the replacement 380934-S36 is an 8-18X1/2pan head, but that is too small to be used in a #10 hole so I consider it to be an error for 64-66, although is was used in 67 on.
Jim