Elgin never made pocketwatch cases. Until well into the 1920s, it was a common practice for a shop to offer a selection of watch movements, and a selection of watch cases. A customer would pick out a movement and a case separately and the watchmaker or jeweler would assemble them together.
This is true of most early American watch makes. And vintage American makes are remarkably standardized in their form factors, so as to fit a great variety of likewise standardized watch cases made by many case manufacturers.
There is an interesting aside about this. Since both movements and cases came at a variety of price points, it is not unusual to find high grade, expensive, movements in very plain cases, or to see a basic, no frills, movement in a fancy gold case that likely cost quite a bit. It all depends on how the original buyer budgeted.
Another interesting historical note is that there was at least one early American watch company that tried making its movements in a non-standard size. Naturally, they made watch cases that fit their watches, so one would have to buy from them. This practice thankfully did not catch on, and these products are quite obscure today.
Hunter and Open-face Movements
Hunting, or hunter, pocketwatch cases are cases with a cover over the front that opens. An open-face case has glass on the front and no cover. Hunting movements have the seconds dial 90 degrees from the winding stem. Open-faced movements have the seconds 180 degrees off the winding stem.
As noted above, vintage Elgin movements are found in all sorts of cases. It is even true that hunter and open-face cases can be mixed and matched with hunter and open-face movements. The hunter and open-face designation on the serial number look-up page refers to the watch movement, not the case style any given watch may be in.
Find more here...
The above comments about watch cases also apply to wristwatches. The Elgin movements in early wristwatches are simply the smaller pocketwatch movements, typically zero size and below.
Watch size numbers represent larger watches as the number increase, that is a 12 size is smaller than a 16 size. The largest Elgin watches that are common are 18 size. The smallest Elgin movements that follow the same convention is a zero size.
Going smaller than zero size, the convention changes. Sizes are written as 6/0, 8/0, 10/0, etc. (or sometime with a dash, like 6-0, 8-0, etc). These sizes get smaller as the first number gets larger. And 8/0 movement is smaller than a 6/0 movement.
Read about watch movement sizes here...
7 jewels, 15 jewels, 21 jewels... Jewels in a watch are not there for decorative purposes, nor or these jewels themselves significantly valuable. The jewels in a watch are mostly used as the barrings of the pivots of the gears, or wheels. The hardened steel of each pivot inside a donut-shaped synthetic jewel is extremely long-lived and mechaniciall reliable, so long as the pivot points are clean and lightly lubricated. Jewels in the shape of pins and posts are also used in the escapment.
Jewel counts in the vast majority of vintage watches work fairly simply. There are two jewels used as pallet stones, and one roller jewel, in the escapement, for three jewels. And three is a odd number. Add to that 2 jewels for the balance wheel pivots, and two more cap jewels to cover each balance pivot, and you get to 7 jewels, which is about the minimum.
The balance wheel is the part whose free movement is must critical in the watch, so Elgin movements always have at least 7 jewels. Going from there, other wheels in the train may also have jeweled pivots, upper and lower, so as to increase the jewel count in pairs. In high grade watches, additional faster moving wheels, such as the escape wheel, may also have cap jewels adding, in pairs. 23 jewels is the most you'll find in a vintage Elgin, railroad grade, watch. And that's realy the most that make any sense.
Watch jewels are typically sythetic garnets, rubies or sapphires.
More about watch jewels here...
And also here...
In each Elgin movement size, there are several model numbers. The model numbers refer to the basic design, such as the type of plates, open-face or hunter case movement, etc. Model numbers are unrelated across watch sizes (two watches of different sizes that are both model 8 have no connection to each other). The same model numbers are re-used across movement grades and classes.
The class groups grades by general quality, materials and finishes. There would be several grades with class 91 for example, which would each be made with a similar finish, use of gold or brass, blued screws or not, and other details.
To my knowledge, Elgin stopped using the model and class designations in 1914.
The watch grade is more widely documented, and best defines the watch. The grade is most useful in looking up the specific replacement parts.
All of these numbers, model, class and grade, have no general organizing principle. They were assigned by Elgin sequentially over time, as the products were created. There is nothing to say that one is "better" than another because the number is higher or lower.