Thin-o-matic T-400
1959–1962 · 10k YGF · Automatic
About the Thin-o-matic T-400
By the late 1950s, Hamilton was pushing toward thinner automatic movements, and the Thin-o-matic T-400 was one of the first expressions of that direction, produced from 1959 through 1962. It runs on the caliber 620, 663, or 666, each a 17-jewel automatic, with the 620 being Hamilton's micro-rotor movement built specifically to keep the overall package slim. The case is 10k yellow Gold Filled and round, with a silver dial carrying what Hamilton described as an "alternate Masonic dial" layout: Arabic numerals oriented vertically in an exploding arrangement, luminous dots around the outer perimeter, and a pearled track inside the numerals. The T-400 is part of the broader Thin-o-matic family, which also includes the T-401, T-402, and T-450.
The dial on the T-400 is genuinely unusual and worth studying carefully when you find one. The exploding layout with vertically oriented numerals and the pearled inner track gives it a lot of visual complexity for a round dress watch, and original examples with clean, uncrazed dials and intact luminous dots are harder to find than the production run might suggest. Gold Filled cases from this era wear well but can show brassing at high-contact points, so check the lugs and caseback edges closely.
From the Catalog
Hamilton originally listed the T-400 in 1959 at $100.00 on bracelet and $89.50 on strap, and later catalog copy called out the 10k yellow gold-filled case, the distinctive Masonic dial layout, and waterproof construction as the watch's key selling points.
Hamilton Catalog Images
1959
1960
1961
1962
What to Look For
| Dial | Look carefully at the luminous dots around the outer perimeter. On examples this old, the lume can shrink, flake, or discolor unevenly. Also check the pearled track and the embossed numerals for any signs of refinishing, which would flatten the texture and reduce collector value. |
| Crystal | These typically came with a plastic crystal. Check for deep scratches or crazing, and confirm the crystal is properly seated with no gaps around the bezel edge. |
| Movement | If the watch has the caliber 620 micro-rotor movement, listen for any grinding or irregular winding feel. The micro-rotor can ride up on its axle and contact the case back, and the wheel arbors in this family of movements are long and precise, making reassembly after a service genuinely demanding. Ask when it was last serviced. |
| Case | Check the lugs and caseback edge for brassing, where the gold fill has worn through to the base metal. Some wear is expected on a watch this age, but heavy brassing at the lugs affects both appearance and value. The swept lug style can make touch-up attempts obvious, so look closely. |
| Crown | The crown should seat firmly and feel positive when pushed in. A loose or wobbly crown on a watch that was marketed as waterproof suggests the tube or stem may have issues worth addressing before the watch goes back on a wrist. |
The Movement
The T-400 was fitted with one of three related 17-jewel automatic calibers: the 620, 663, or 666. The caliber 620 is the most architecturally interesting of the three, using a micro-rotor rather than a full peripheral rotor to keep the movement thin enough for the Thin-o-matic brief. The 663 and 666 are more conventional automatics but share the same general slim-profile philosophy. All three wind automatically with the motion of the wrist and drive a sweep seconds hand.