Got curious and checked the Branda idler arm I bought in 1995, the seal does have "D7DC-3A196-AA" molded into it. Interesting that the arm JD Larson was selling on ebay, which is now sold, has a seal marked "ZA-3A196-A", so possibly that has a C4ZA-3A196-A seal where the C4 is very faint?
Also, the OE idler arm photo I posted in reply #19 has a seal with no numbers at all, I said it was from a later 66, but checked my notes and it's actually off a car in the 300's, but is identical to the original arm for 6S1757.