An example of evolution in watch movement design. How movements can change over decades. Our subjects are: a later grade 950B Hamilton on the right, and an earlier grade 950 Hamilton on the left.
1- At A on both images, the arrow points at the balance wheels on both movements. On the grade 950 (left), we have a bi-metallic, temperature compensating balance wheel with blued steel hairspring. On the 950 B on the right, the arrow points at a solid rim, mono-metallic balance wheel, and an alloy Elinvar Extra hairspring.
2- At B on both images, the arrow points at the fourth wheel upper jewel. On the grade 950 on the left, the jewel is in a separate setting, fitted into a milled recess, and held in place with two screws. All the jewels on this grade 950 are this way. On the grade 950B on the right, all the jewels have adopted the Swiss system of friction fit, non-removable jewels.
3- At C on both images, the arrow is pointing at the regulator whiplash spring. On the grade 950 on the left, the whiplash spring is the same spring as used on (for example) the grade 992. On the grade 950B on the right, the whiplash spring is the same as the grade 992B.
4- At D on both images, the arrow points at the barrel bridge. On the grade 950 on the left, the arrow points at the split in the barrel bridge, making the barrel bridge into two parts. On the grade 950B on the right, the arrow points at the same spot. On the 950B, Hamilton made the barrel bridge a one-piece bridge. Their reasoning was that the one piece bridge was simpler to work with when the barrel bridge had to be removed, say, when replacing a mainspring.
Other differences include changing the location of the case screws. Fancier etching on the ratchet wheels and crown wheels. Gold train wheels on the grade 950, and brass train wheels on the 950 B. Chatons on the jewels of the 950 are gold, and brass on the 950B. Note the longer neck on the pendant of the 950 compared to the 950B. On the 950, the upper cap jewel on the escape wheel arbor is inset while the upper escape wheel cap jewel on the 950B adopted the same method as the 992B. The cap jewel is in a separate bib, affixed with a screw. Compare the different barrel bridge shape at the edge of the ratchet wheel on the two models. Slightly changed shape of the train wheel bridges on the two models.
The grade 950 (left) was made about 50 years earlier than the 950B. They are both collectable watches, and in their day, they were equally capable of doing the job they were made to do. But many folks prefer the greater attention to detail on the grade 950.