Ok, here we go...
Keep in mind this is just 66/67 289 anti-backfire valves except intake manifold type:
Of nine "C6OE-9B289C-L3554" valves, eight are mounted on C6OE-B (with extension) brackets. legible date of Feb 17, 1966, what looks like Jun something, 1966, and another illegible ink date stamp. The ninth is mounted on a C6OE-D bracket with a stamped date of Mar 11, 196x (can't tell if it's a 6 or 7).
"C6OE-C-L3554" valves: any still on brackets are mounted to C6OE-D brackets. Dates: 693,602,6A1,6B2,712,714,724,732,734,764,784,943. All of these have "Fomoco" on the top, I'm not including any stamped "Holley".
From this info, it would appear that Carter was the first supplier to Ford, based on the earlier ink-stamped dates (including the C6TE valve with an Aug 30, 1965 date i posted earlier, and an Aug 28, 1965 date on a C6AE-E valve in Bob Mannel's book on page 6K13) with Holley coming in for the '67 model year.
Also, I would guess that the odd Holley dates (602,6A1,6B2) would translate to Oct. 2, 66; Nov. 1, 66; Dec 2, 66. (0=10th month, A=11th month, B=12th month).
Has a date been determined for the changeover from the C6OE-9B290-B to the C6OE-9B290-D valve bracket?