Most searches may have problems since vendors list parts (especially NOS) by the engineering number.
The two port anti-backfire valve used on the 67 390 would have used C6AZ-9B289-C as the part number. Think I listed that number over in one of the responses in the Mustang 390 emissions discussions. If it looks like the pictures I posted and has two ports then the only variations I can think of would be the supplier and when it was made (variations) should be the same
Also same part number West Coast Cougar lists for the application
Goggle or Yahoo that part number and you'll get some hits