bconde219...Hello!
From what I can see here, you look to have a nice watch that may well have been assembled from a few different sources...which is actually something becoming rather popular.
The movement is an "0 Size", Model 1900, 16 Jewel, "Lady Waltham". My reference puts the Serial Number, 12,587,673 at 1903-04. These movements were among the better-Grade Walthams, and it's unusual to see a 16 Jewel. It's also signed "Adjusted", which only appears on the higher-grade USA made watches. You also see "Raised Gold Jewel Settings", which--once again--only appear on high-grade USA movements. This is a very-good quality Waltham.
My guess is that this "Hunting" movement was originally in a "Hunting Case", the ones with a cover over the front, that spring-open when you push on the crown. There were also "Open Face" movements, and these are the ones where you can just look at the watch, and see what time it is. The case was probably either solid gold or "Gold filled", although it's possible that it was Sterling Silver.
The dial sure looks to be from a different Model. Waltham tended to match dials with movements--Style-wise--and I rather doubt but that a strong, 'Railroad style' dial would have been thought to stylistically compliment something as delicate as a Lady Waltham. Same thought @ the hands: they look way too aggressive for the Lady!
The case has had another Movement in it ( note the semi-circular indentation over by the balance ), and I'll assume that it was one of the Open Face Movements that Waltham made in this size, although it's entirely possible that another Company's movement was there...a number of USA watches were made in 0-size, and they, too, would have fit in this case.
So: to my mind, the person who put this watch together did a good job! The hands 'match' the dial, and go well with the case. I like the crown, too. It's large enough to be practical, and goes with the rest of the watch. The movement is really nice & unusual, and should be a fine timekeeper. I know that there are many, fine-quality movements out there that have lost their cases: when Gold shoots up in 'value', many gold & even gold filled cases perish. This movement has been saved from Oblivion by being 'repurposed'...different dial and hands / slipped into a good-looking nickel case / fit with a proper crown...Voila!
Finally: if you'd ever really want to, this Lady Waltham could be 'put back together' pretty-much as it was when new. A gold case, bit fancier dial & hands, and you'd have a very elegant, 0-Size, Gold Hunting Case pocket watch. These components are available, and not really all that hard to find.
Still...to my eye, your watch looks very good just the way it is. Enjoy!
Michael.