Ahhh....the common English gin. And no, not the stuff you drink...elixar of the gods, in my opinion.....but rather the common English rat trap. These were used for trapping rats, Norway rats, to be specific, but were found to be just the thing for muskrats as well. Single sprung, with either a flat spring riveted to the base, or a V-spring, sometimes with the Y-post, but quite often with the jaws just pivoting in holes in the upturned base. As Mike mentioned, quite often they'd have one end of the jaws pivot in the upturned base, while the other end of the jaws had a Y-post. There's alot of different ways to make these. They should be much easier to rework from a modern trap than the larger beaver traps, as the manufacturing style hasn't changed quite so much.
Changing the pan, and rechaining a modern single spring muskrat trap will be pretty close. One thing about a V-spring, as well, is that they are easier to use in a set. You can 'crook' the set spring over to 75 to 85 degrees in relation to the base, so it will sit better in the set. Same goes for beaver traps. I can imagine the difficulty using the beaver trap that Isham illustrates up at Hudson's Bay--the thing would be about three feet long! Try setting that under a slide---I'd think the only way to use it would be set lengthwise in a breached dam set.
Rod