I watch these all the time. The Hitchcock was a good one.The Gleason one was very useful to me in my research - especially to watch his instant recovery choice when the chalk breaks from his typical hard pressure. Often he sliced the paper with the nib.
That's my favorite part of the Mystery Guest segments, watching them actually sign their autographs. I hate it when some celebrity non-mystery guests are only identified as "Mr. X", and only sign in as such, because their names may be known but their faces aren't. For example, Beatles manager Brian Epstein - 2:58 at the linked video. (What's My Line, October 18, 1964)