In 1975, a short, 13-episode radio series of the Fantastic Four was produced. Voice actor Jerry Terheyden voiced all the villains in the series, from Dr. Doom to the Puppet Master. I found his home address, and sent him a LOR, 4 FF cards, and a SASE on 3/21. Today, I received everything signed along with a nice thank you card in his own envelope:

I think he used a dry erase marker, because everything (especially the Mole Man card) is REALLY smudged (on Mole Man, you can barely make out his name). His thank you card was very nice, though, and made me go "awww."