In “addition” to these sizes, 18-s, 16-s, 12-s, 6-s, and 0-s; Seth Thomas manufactured some “compact” models and some “snug” models. 

Some of the 6-size pocket watch movements were manufactured with a longer setting lever, enabling them to accept the same size dial as is used on a 12-size watch.  The later 6-size pendant set movements did not need to be modified at all to accept the larger dial. These watches are referred to as 12X6 movements by collectors and watchmakers. They were marketed to the public as thinner, less bulky 12-size pocket watches. Most pocket watch dials are slightly smaller than the pillar plate that they rest upon to reduce chipping and breakage of the enamel dial; so a dial for many 12-size watches measures only about 10-s.  The dial on the 12X6 movements are 10-s, just like the 12-size movements; but they protrude past the 6-size pillar plate.  Specially adapted 12-size watch cases are required to accommodate the 12X6 movements which are really just 6-size movements with a longer setting lever to extend beyond the edge of the oversized dial. The resulting benefit of the 12X6 movement is really the ability to sell the 6-size watch in a somewhat compact 12-size watch case to people who don’t want to read the smaller 6-size watch dial.

You can notice in some advertisements for the Centennial grade watches that a “4-size” watch is listed.  In other literature you can notice that a 6-size “snug dial” is referenced. These two phrases are connected in that the Seth Thomas “4-size” pocket watch is really their 6-size pocket watch with a “snug dial” and a “snug case”.  The snug dial is the same size as other 6-size dials; but its painted features are moved closer to the center, away from the perimeter of the dial, and its seconds bit is smaller. The watch movement with the “snug dial” is then placed into a slightly scaled down 6-size watch case with a smaller exterior dimension, a smaller bezel opening, and a smaller seconds hand; making the watch appear to be a true 4-size watch.