I was looking at a Vacheron Constantin watch described as a "tank," and so I looked up the movement, VC cal. 1003 on the main Vacheron Constantin site. To my surprise, they are still using the same caliber albeit as 1003/3. Even better, it carries the Hallmark of Geneva. I learned that after the initial build of the movement, it was revised with a "shock absorber,--without increasing its width" and the original 1955 c. 1003 was the smallest and slimest caliber of its kind. (The 2013 version of the Prestige of 1972 runs on the 1003/3.)
In a 2007 discussion on The Hour Lounge Alex solves the mystery I encountered, but after nine years, I think it's an interesting story; approached from a slightly different direction. As I said, I found a watch I liked and so I began investigating its background. I found a 1003 movement, but it did not have the Poinçon de Genève as did the 1003/3. I just assumed that by the third version, it has been upgraded to meet the Geneva Hallmark standards.
Calibers 1003/3 and 1003
So I did some more searching and shortly I found two more caliber 1003 (version 1) with the hallmarks.
Caliber 1003 with Poinçon de Genève
So my first assumption was that there was some monkey business going on. I assumed (wrong I learned) that the 1003 without the Poinçon de Genève was a fake and the counterfeiter screwed up and didn't know enough to add the Geneva Hallmark. The older 1003 movements have the tell-tale "&" between the Vacheron and Constantin, and the original 1003 certainly had that (even though the 1003/3 has a Maltese cross instead).
Original 1003 and Geneva Seal
However, since the 1003 image without the seal was from The Hour Lounge, I was pretty sure that someone had a question about it. Sure enough there were some questions, and Alex learned from VC that back then (1955...) the seal was not seen as a big deal, and sometimes it was added to a bridge on the movement and sometimes it wasn't.
My grail watch is still the Overseas 4500V/110A-B128 with the caliber 5100 movement (which does have the Geneva seal), but since my other two VCs are both based on other ébauches (c. 1311: Girard-Perregaux 3100--OS1; c. 1050: JLC + Patek), it's be nice to get a VC with the 1003...while I'm saving up for the 4500... The only brake on my passion is...
...the mighty rolling pin.