author=jwc66k
I think an education session is due. From my notes (mostly 65 and 66 San Jose cars), by 1969 the valve cover bolts were grade 5 or 5.1 (three marks at 120 degs apart for gr 5, and one mark at 9 o'clock, one mark at 12 o'clock, and one mark at 3 o'clock for gr 5.1), all had a split ring lock washer attached, all were 1/4-20 thread, and had a length of 5/8 inch, or possibly 3/4 inch. This also applied to the oil pan except for the two on each side at the ends which were 5/16-18 types. At the least, they were clear zinc plated. The head markings you describe are what I've seen plus several more. Unfortunately there is no factory engine assembly documentation available, and the Ford Car Parts service parts are a separate hex screw and washer.
If you could provide a description a bit more detailed, a correct replacement may be found. My own inventory has about 150 grade 5 and 5.1 screws of various head markings that are not used on anuy of my 65 and 66 Mustangs.
Jim
Re-visiting this thread and can add a little more information. Confirming that my 65 oil pan and valve covers take the 1/4-20 X 1/2" bolt and uses a 7/16" socket. The two forward bolts, at 65 timing cover, take the 5/16-18 x 5/8' bolt and uses a 1/2" socket.
The 69 uses the 5/16-18 x 5/8' bolts at the valve covers and the oil pan. There are no grade 5 or 5.1 bolts installed in any of these locations. The predominant bolt in use is the ' B&H ' unit with lock washer. Brian