Here's a screen shot from a 1971 Challenger road test I found on youtube, you can just make out the bakelite valve.

video-
https://www.youtube.com/watch?reload=9&v=zbzrxowPjZw

And the other pic is of a 1970 383 R/T SE Challenger that just surfaced over in England on another board, it's been off the road since the late 80s according to the guy who found it - looks really original under the hood, '171' master cylinder and all.

1971 road test.jpgpurple 383 rtse in england.png