The Model 21 was the company's only known 16-size movement, and it came well after most of the other American firms had launched their own 16-size designs. It was part of the Centennial Series, introduced around 1913 to mark their 100-year anniversary, and offered in three jewel counts on a single nickel pinstripe pattern. All three of the Centennial sizes within the grade were retailed in either nickel or 10/20-year gold-filled factory cases.
* This is an assigned model number. The factory designation is not known.
Roughly 62,000 were produced in two separate serial number blocks, with the first larger run starting at SN 1,304,501 up to 1,360,000, and a second smaller run beginning at SN 2,593,001 and ending at 2,600,000. The reasons behind the existence of a second block are unclear, since the first one could have continued all the way to SN 1,700,000. One explanation is that the Model 21 was made at different times and therefore blocked separately, but the only catalog known to advertise the Centennial Series was the 1913 edition, with no mention of any 16-size models in the 1909 volume. No private label examples have been reported.
The first known appearance of the Centennial Series was in the 1913 Trade Price List, which would make sense given the company's established founding date of 1813. With the two distinct serial number blocks it's likely that the actual debut was a little earlier, though a design date or granted patent number is not known. The three models of the Centennial Series were the only pocket watches to be listed in the 1914 Catalog supplement.
There are three known Model 21 grades. All of them are open-face with a simple pinstripe pattern on nickel plates.
* The 17-jewel Grade 336 is assigned.
The Centennial Series was a named grade produced in three models, including the 16-size Model 21 family. While the standard Seth Thomas movements have three different jewel counts with the higher counts found only in the second smaller run, the Centennial grade had 7 jewels and have been reported only in the first serial number block.
The Tourist was a newer, smaller version of the 18-size folding travel Companion, containing a 7-jewel Model 21 with all of the standard features like a safety pinion and a Breguet (overcoil) hairspring on the balance wheel. It was offered in several different leather color choices, selling for $7.25 in the 1914 catalog. Note that the movement was upside-down, winding at the 6:00 position with no seconds bit on the porcelain dial.