Quanto al significato della sigla SOB, che identifica il tuo e il successivo 26.5, l'interpretazione prevalente è "Savonnette Original Base" (piccoli secondi al 6, diverso dai calibri Lepine; altezza non ribassata; regolazione non cronometrica).
Da WATCH POP:
S indicates savonette (French for hunter), meaning the relationship of the crown to the subseconds dial is 90 degrees apart, rather than opposite, as in hunter cased (covered) pocket watches, as is traditional for wristwatches, crown at 3, subseconds at 6. Open faced pocket watches have the crown at 12 and subseconds at 6, an arrangement referred to by Omega as Lepine (L). As it was the standard arrangement for pocket watches, it was generally omitted as it was the default.
O indicates it is the Original thickness of the movement where variations were available; other indications were P for Plat(French for thin).
B indicates the movement is of base quality, where higher quality variations were available.