The pen manufacturers often used numbers to describe their pens mainly as a way for the dealers to order them - Parker used language like Senior, Junior, and Lady but some of the others like Waterman, Mabie Todd and Wahl used numbers to describe all of the variations in size and metal ornaments. The Mabie Todd catalog from 1921 was 37 pages of different pens styles, and it would have been difficult to brand them all anyway (talk about confusion!).
Mabie Todd introduced the lever filler sometime between 1910 and 1920 (I forget exactly when Cob may know) and called it the "Self-Filler" - "SF" is short for that. The predominant filling system during that time was the eye dropper, so this was a big innovation. Some pens are labeled "SF" and the British subsidiary (and ultimately its own company) continued to use "Self-Filler", "Leverless" and other descriptive phrases. The American pens in the 1920's tended to stick to numbers.
The "Eternal" line was introduced in 1924 (that's what all the literature says, but I'm skeptical. I think it came in a few years earlier) by Mabie Todd to compete with the likes of the Sheaffer "Lifetime" and the Wahl "Signature" pens. It had a heavier slab of gold than the original pens. However, they continued to make non-Eternal pens as well. The funny thing is that while the Eternal pens were considered their high-end at the time, the thicker gold often means that the nibs are less flexible, and are now less desirable than the non-Eternal nibs. The much harder to find 6 and 8 size non-Eternal nibs are considerably more interesting to write with, although the pens themselves (being the lower end of their line) are not as interesting. Eternal pens had the marking "ETN" under the model number.
The first number, "4" (or "5" or "7") was the description of the line of basic self-fillers. The nibs ranged from 2 to 8 in size (although the British had 1 size nibs as well).
Some of the earlier pens that were the pre-cursor to yours were marked SF-2, and it was later that they went to a "4" to reflect a self-filler without a cap band. They used a "6" for a pen without a clip. They used a "1" for a screw-top eyedropper (they called it a "safety" because the inner cap met the lip of the section causing a seal so the pen wouldn't leak)
Here are the prefixes in the 21 catalog:
1 - Eyedropper with a screw on cap and a clip
2 - Eyedropper with a screw on cap and no clip
3 - Eyedropper vest pocket without a clip
4 - Lever filler with a clip
5 - Lever filler short length but the wider version
6 - Lever filler without a clip
7 - Lever filler short length but thinner
Ornate cap band - add a one in front
Gold plated - add a letter in front (depends on the style)
Got it? A clipless eyedropper with a 6 nib would be a 26.
Anyway, yours is a very standard 42, the most simple of the pen descriptions.