Here we have a vintage Hamilton Thin-O-Matic Model 400-A from the Masterpiece series featuring a self-winding 17 jewel ultra-slim 10K yellow gold-filled case. The dial consists of numerals, markers, and a pearled track with the Hamilton logo and Masterpiece underneath the 12 and Thin-O-Matic printed above the 6. The case measures approximately 33mm wide x 36mm from lug end to lug end. Powering this watch is a Hamilton grade 623 automatic movement with 17 jewels that has been serviced and kept good time over a 24 hour period. The inside back case reads: Hamilton W. Co., Lancaster PA., 10K Goldfilled, R530837, Star W.C. Co., 993166.