Saw one on a 1940, 4-dr. Ford, back in the early 70's. Was pretty nice. If I'm not mistaken, the guy had clear valve covers too. Haven't seen them since on anything.In my 36 and 1/2 years of hot rodding, I have never seen a clear cap installed on car. The first few years I was too short to see over the fenders so I could have missed one.
I had that setup on my 62 Buick Special back in the day! Bought the transparent red plug wire on a spool and made my own plug wires! Good thing I got a spool that wire didn't hold up to heat very well and they would burn up frequently! The clear cap held up very well but tended to attract a lot of moisture!I have seen them on rat rods with the old clear red plug wires.