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Last I watched the I've Got a Secret video.from '62. I counted three shots where he either did a last minute grip adjustment, or perhaps a slip stroke. On four shots he does not. And on eight shots you can't tell. On the two trick shots he shoots (six-on-one and the RR Shot) he does not use a slip stroke.
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Lou Figueroa
Gee, Lou, I count only two shots in the I've Got A Secret video where you can clearly see his backhand from the beginning of the backstroke to the end, and both of those showed the modest slip (these were the break shot and the one following). I count two more where you can see his backhand but only on the forward motion of the final stroke and I can't tell whether he slipped or not - they were short shots and maybe he didn't. In some other shots I tried what you must have done, to see whether he ends up further back on the wrap than where he started but I can't reliably tell, since his slip, if it be such, is so modest.
I think I originally got the idea of Mosconi slipping from a George Fels column years ago called, if memory serves, "Slip Stroking Away" or something close to that. George discussed slip strokes in general and the men who used them, among whom he counted Mosconi.
And, yes, I saw him in person twice but I was just a baby. I didn't see a slip stroke then but I wasn't looking for it. Mainly I saw the back of some tall guy's head (tall guys - I hate 'em).
Rich