Hi,
AP has traditionally used mainly JLC, Valjoux, and Piguet base movements.
BUT...depending on how strictly one wants to define what belongs to whom (including source of inspiration for ideas) as well as how one wants to define "ultra thin" then AP can claim to have had its own "ultra thin."
The 2003 ultra thin manual was based on a JLC caliber;
The 2120 ultra thin full rotor automatic was based on a JLC caliber.
AP, VC, and PP funded the JLC development for the 920 base for the 2120, itself the brainchild of a Belgian (?) watchmaker, and was used exclusively by its underwriters.
The 2003's base movement was also exclusively used by AP and VC. What is mostly unknown is that a case can be made that the base JLC movement was inspired by the 9 ligne LE Piguet cal. ML that belonged to AP before the 1940's, when the development for the AP/VC cal. 2003/1003 was undertaken by JLC.
Walt Odets is a brilliant auto didact and writer and I have tremendous respect for his work, especially given the limitations he faced when he authored his articles, but his history of the industry in those original articles (unless updated since) were based on several degrees of separation from those who actually had direct archival record access.
Another great movement caliber myth, a mistaken repeat of a print source error, is the existence of a variant of the 2120 so called "2122" - supposedly a 2120 equipped with second hand.
A seconds hand equipped 2120 does not exist, regardless of center second or sub-second.
Doesn't exist; never did, and probably never will.
Cheers,
TM