If you've studied Hamilton's 1950's and '60's watches closely, then you've probably picked up on the rationale behind the numbers used when Hamilton named Automatic models. 100-series watches were solid 18K gold. 200-series were solid 14K gold. 300-series and 400-series are solid 10K gold and gold filled, respectively. 500-series are stainless steel and 600-series are rolled gold.
Add to the nomenclature that if the second digit is a 5, then the back is likely stainless steel. So a K-150 is an 18K solid gold watch with a stainless steel back. A K-451 is 10k gold filled with a stainless back.
Makes sense... right?
Well - almost. There's one model that is a bit of a mystery, that being the 1955 Automatic K-375. It was produced for four years.
If you look closely, the K-375 catalog says it's 10K yellow gold overlay. That's the mystery... what exactly is "gold overlay"? You'll see more about that below.
The back is stainless steel. That's not terribly unusual, After all, there are even 100-series 18K watches with stainless steel backs too, for example, the 1955 K-150.
Like most high-end Hamilton watches, the K-375 offers solid 18K gold numerals and markers on a silver dial.
Behind the dial is a Hamilton 661 automatic movement.
I recently picked up a K-375 project watch. It arrived in decent condition, just a faint radium burn from the lume on the hands. I suspect the watch was tucked in a drawer for quite a few years, stuck at about 5:45.
The back of this watch has two parts; a pie pan-shaped cover held in place with circular ring.
Here's the second part of the mystery... the inside of the case back is stamped "10K G.F. Bezel" Why would Hamilton stamp the inside of the case back gold filled, put gold overlay in the catalog and name the watch a 375? Why not make is a 400-series watch? Weird, huh?
The best I can guess, and it's a total guess, is that gold overlay contains substantially more gold than gold filled. Gold filled usually implies about 5% of the thickness of the material is a layer of gold, while the remainder is a base material (brass, or some other alloy). When you see "1/20 gold fill", the 1/20 implies 5%. If it was 1/10, the gold content would be 10%, etc.
I just overhauled a 661 in a K-250 yesterday, so I'll spare you the details again. However, this one was a little more challenging as it took me several attempts at cleaning the balance assembly before I could get it to run cleanly, as shown below on my watch timer. A clean-running watch is represented by two, fairly close, parallel running lines. Ideally the two lines would be horizontal. Trending upwards means the watch is running fast... in this case 27 seconds per day fast. I find recently overhauled watches will settle down after a little while, so I leave them running slightly fast.
With the dial and hands reinstalled, I can put the reassembled movement back in the case. The oscillating weight, aka rotor, can go back on now. One additional insight into the mystery of the case material is that there is absolutely no sign of verdigris on the case. Verdigis is the green "rust" that you see on gold filled cases where the base material comes into contact with moisture. Solid gold cases never show verdigris since only gold is present. This watch's case is a clean as a solid gold case.
Although the Speidel bracelet isn't original to the watch, I don't have any 18mm straps at the moment to install on it. So the bracelet will have to do for now. It looks pretty nice though - so maybe I'll keep it. I think the watch cleaned up very nicely. I removed the radium from the hands in my ultrasonic cleaner so there will be no more damage to the original dial.
The K-375 is a sharp looking watch, don't you think?
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