I'm sort of a latecomer to What's My Line? fandom, but I've really gotten into it over the last few years.
Yesterday, I was brainstorming an interesting question - who are the celebrities who appeared as both a panelist and as a Mystery Guest on both versions of the show? I decided to do a little research and see what I could find.
Surprisingly, it's not an especially big list. Most of those who you'd think would make it were felled by one of the four - usually, being an MG on the CBS version.
Here's who is confirmed as hitting the WML? Grand Slam, in no particular order:
Dina Merrill (who was the celebrity who got me started on this)
Gene Rayburn
Jayne Meadows
Steve Allen (as far as I know, he's the only regular panelist that did this)
Betty White
Henry Morgan
Garry Moore
Eamonn Andrews
Mark Goodson
Sheila MacRae
Mel Torme (probably the most surprising entry to me; he only appeared on the CBS version twice, once in each category)
Peggy Cass
I did count group MG appearances towards this, as well as Eamonn Andrews' appearance as a "regular" contestant.
The following three men all fit three of the four categories, though I do not see a confirmed syndie MG appearance for them:
Soupy Sales
Robert Q. Lewis
Allen Ludden (paging Adam Nedeff)
I also did not see a syndicated panel appearance for Kitty Carlisle - does anyone know if she ever appeared?
Unsurprisingly, most of these people were game show personalities, either as host or regular panelist on another show.