In 1955 Hamilton added 7 more models to the automatic lineup, including a couple of 18K models in the line K-100 line. 14K models were in the K-200. 10K models are 300 and gold filled are 400 series models. Stainless steel models were given names in the 500 series.
One of the 1955 introductions was the Automatic K-503. It was produced for a long time (at least by Hamilton standards) - all the way until 1960. There are no serial numbers inside the watches to date the year a K-503 was produced, but slight changes to the dials occurred over the years so you could actually collect four different varieties of K-503.
In the 1956 catalog there were black and white versions of the K-503. In 1955, only the white version was available. Note the direction and size of the triangular hour markers, they both point outward but the black version's are smaller.
Behind the dials you'll find Hamilton's 661 movement. It was used predominantly in the K-series watches. When the Accumatics, Thino-matics, Kinematics and Rotomatics were introduced, so were new movements.
I recently picked up a nice black dialed K-503 so it's probably from the 1956 - 1959 period. I almost bought a white dialed version too - but the dial was different and I (wrongly) thought it was a franken so I passed on it. I know better now - and so do you.
My black-dialed version is definitely in need of a new crystal and an overhaul.
The screw-off case back is clearly marked K-503.
Pretty good time keeping fresh out of the gate. Vintage watches can often keep excellent time but I think a range of +/- 30 seconds per day is a well running watch. +/- 1 minute per day is fair too - beyond that a little tweaking or an overhaul is probably warranted.
Here's the movement all cleaned up. I find the difference between a cleaned movement and a dirty movement to be visually very satisfying. When it runs as well as it looks, it doesn't get better than that!
And here it is all polished up with a new crystal, new strap and fresh lume applied to the hands and the dial hour markers.
The K-503 is a great looking watch and I can see why it was produced for 6 years.
UPDATE - DEC 17, 2014
Here's a 1957 white dialed K-503 I restored, a sibling to the black dialed version above.