I have a couple of both. I know there were a great many more 940s made than 942s. My watchmaker friend had promised to show me but he passed away before we had a detailed discussion.
Thanks,
Rick

Richard,

Here is a 942 from my collection

The dead give away of the 942 movement is the wavy (ribbon) demaskeening pattern around the outside edge of the movement plate. In the provided photo - if you look where the serial # and adjusted are stamped on the movement you can see the ribbon clearly. This pattern is not found on the 940 (an example pictured below)

And a 940

Jon look at where the banking pins are at on the 942.

I'm guessing the the 942 has a completely different pallet fork assembly.

Based on the movement pictures I have, there was a change to the pallet/banking pin configuration on later 940's. The ones before around sn653000 had the same banking pin location as the 942. One other obvious difference between the two is the flat head plate screws on the 942 vs the domed ones on the 940.

Thanks guys. You give great info. I love the 18s hamilton's. I tend to have more 942s, 944s than 940s. thanks for the help.
Rick

i think you might find Gene that the pallet forks assemblies are not that different unless the 942 was made with single roller. The position of the banking pins on the 942 have just been moved to the back of the fork, this is seen on a lot of earlier full plate watches (942 in the photo is much earlier serial# then 940). The 940 has them just behind the yoke, positional difference only and still can have same fork design.

Cheers,
Bila

The 21J Grade 942 always had a double roller escapement, as did its 17J Grade 938 predecessor.