The Y513/4 have been used as a swap movement for 754X's and are also down jeweled to 2 jewels - so probably where Micah remembers the 2J from?
The difference in thickness is probably down to Seiko using measurements sans battery with the 754X's, whereas the Y513/4 height is not noted as measured without battery.
The Y504 shares the same battery, coil resistance and current consumption as the 754X's - so I'd hazard a guess that it's largely interchangeable with the Y513/4 and 754X series.
Many divisions and companies have been absorbed into the Seiko family, and they seem to use all of them in some way - eg Hamazawa, Shiojiri, Hattori, Morioka, Epson,Tokei, SII, Alba, Lorus, Pulsar etc. http://www.thewatchsite.com/21-japa...-forum/109842-hattori-vs-seiko-movements.html