That statement about Mosconi challenge matches is easily checked, thanks to Charlie Ursitti's compilation of all 14.1 championships and championship challenge matches, i.e., challenges where the winner becomes the official champion (
www.charlesursitti.com). And it turns out to be quite wrong. Mosconi won 12 of his 13 championship challenge matches, losing only his first challenge match to Andrew Ponzi. Neither Crane nor Caras ever beat him in a championship challenge match. Some of the margins of victory are embarrassing, especially during the latter period but before his stroke. Conclusion - in a long match Mosconi was pretty much unbeatable during his prime.
Here are all the championship challenge matches Mosconi played in during the '40's and '50's. In order to challenge for the championship title, a player had to have "standing" (most likely, being the runner-up in the previous championship tournament or a prior champion). I'm quite certain that Cranfield never had such standing during the Mosconi days. That's not to say that they never played a challenge match, just never one for the title.
April 13-17, 1943... Ponzi 1250, Mosconi 1050
Feb 29-March 4, 1944... Mosconi 1250, Ponzi 924
Jan 29-Feb 24, 1945... Mosconi 5498, Greenleaf 3738
Feb 4-March 30, 1946... Mosconi 8727, Caras 7508
Nov 6-22, 1946... Mosconi 3750, Crane 2919
May 8-18, 1947... Mosconi 2000, Crane 918
Oct 25-Nov 12, 1947... Mosconi 4000, Caras 2334
Mar 5-7, 1948... Mosconi 1350, Ponzi 643
Jan 8-19, 1951... Mosconi 3000, Crane 2323
Feb 22-Mar 11, 1954... Mosconi 2400, Procita 989
Nov 28-Dec 2, 1955... Mosconi 1500, Crane 676
Jan 30-Feb 22, 1956... Mosconi 6300, Caras 3007
March 15-20, 1956... Mosconi 1800, Jimmy Moore 879