Or did they sell raw ebauches to other watch makers? I came cross this slightly obvious Longines, marked Cal.1292 but with no serial number and marked S&Co. There are a few examples of this Longines calibre around the Internet, both with and (rather surprisingly) without serial numbers. But I hadn't seen a Longines marked S & Co. [Stauffer, - which I am not trying to pretend means IWC] Or should I assume that with this more modern crown, a dodgy dial leg setup, a missing movement retaining screw and yet a third name on the dial (West End Watch Co, possibly with telltale hairlines?), this is just some bodged together marriage? And why on earth would there be so much wear on that wrong upper-positioned screw on the central bridge?
Rather than "S & Co.", the marking by the balance is probably "B & Co.". The "B" stands for Baume, the British importers of Longines for many years. These "B & Co." movements often had no easily visible serial numbers so the number on the case is to be taken as the serial number.
I Don”t know about selling ebauches to other makers, but I frequently run into private label marked Longines movements with no Longines identification on them.