This has been an ongoing mystery among Wheel fans. Nobody can find the font used for the letters on the old puzzle board (before they switched to an electronic one in 1997). The only other place I've seen it was on the play money on one of the home game adaptations, but never anywhere else significant.
Here's a collage of every letter taken from various older episodes on YouTube... (you many need to click it to view the whole thing.
And here's a normal screenshot in pretty good quality...
It looks like it could be a derivative of Beton. The closest fonts I've found are Beton and Rockwell, though something that stands out to me are the serifs on the E, T, and F. Notice how they slant outwards. Beton and Rockwell's serifs are straight.
Here's a video where you can see some letters in very high quality: