My opinion as to whether Willie Mosconi used a slip stroke is based upon seeing him in person five times, give or take a time, in the late 60's early 70's. This was watching him play a match to 125 and then doing trick shots. My recollection is that I never saw him use a slip stroke.
I very quickly, on fast forward, just watched three videos, which I believe most of you can access one way or the other.
First, I watched Mosconi vs Caras at the Lakeshore Athelitic Club in Chicago in 1963. Caras wins 125-26. In that video I counted 18 shots where Mosconi was definitely not using a slip stroke or late grip adjustment. Six shots, where because of the camera angle, I couldn't determine what he did.
Next, I watched the Mosconi Straight Pool Break Shot video from 1980 in which he runs 28 balls. On 20 shots he does not use a slip stroke or last second grip adjustment. On seven shots the camera angle does not allow a determination. And on one shot *maybe* he does a last minute grip adjustment or slip stroke.
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.
So what do you have after all that? Three, perhaps four shots on TV -- in a non-competive setting -- in which, maybe, he used a slip stroke and 42 shots where he didn't and another 21 where we can't tell (but he probably didn't), plus my recollection of his shooting over 600 balls in competitive exhibition play and however many times Dale saw him play, from which neither of us recall a slip stroke.
I don't know what was going on those three shots Maybe it was a new cue or he had just got it rewrapped and it was slippery, or the TV lights affected him, or he was just goofing. We dont know. But from the vast preponderance of evidence -- video and eye witness -- Willie Mosconi did not use a slip stroke.
Lou Figueroa