Looks like the one on the left.
OEM # is 2466548 and last I knew, you could still buy these over the counter at the dealer. I see them on eBay too, but usually cheaper from the dealer.
Throttle Lever Shaft Kickdown Seal 727 904 TF6 TF8 Transmission Torqueflite | eBay
Pull the E-clip off and the second seal is under it. Really easy to change. Note this pic has the other seal removed.
As an FYI for everybody else, this is the same seal used in a '65 and earlier 727/904, except it goes in the case rather than the valve body.
