I have always had an interest in these, but recently, these have become one of my major collecting focuses.
The three major "large" size Waltham keywind models are the 1859 model, 1860 model, and 1862 model.
The 1859 model was Waltham's first of this design, and began as a premium model although was later used primarily for lower grade watches. The movement will fit a standard American 18 size case, but is thinner than its contemporary full plate watches(the center winding hole presents an additional complication). My one example of this model is the lowest grade, the Wm. Ellery
The 1860 and 1862 models were designs developed at Nashua, and later produced by Waltham after the failure of the Nashua watch company(for many years, Nashua was maintained as a separate department at Waltham). These watches are essentially the same design, differing primarily in size. Both were produced in three grades, with the American Watch Company grade(19 jewels) being the best, followed by the Appleton, Tracy &Co grade and Am'n Watch Company grades(both 15 jewels). The Am'n grades and AT&Co grades are essentially equivalent in finish, with the Am'n grade simply being introduced later.
Here is an 1860 model Am'n grade.
(I don't have an 1862 yet-I have one in my sights)
With all of these, the movements are relatively available. The difficult part, however, is finding an original case. As mentioned, the 1859 model will fit in any standard 18 size keywind case, however essentially only an 1859 case will be correct for it(the cuvette needs a center hole, and the bezel should NOT be hinged). 1860 and 1862 models will only properly fit in cases intended for these models. In addition, many 1860s and 1862s, as well as high grade 1859s, were originally in gold cases(and heavy 18K ones at that) which have not fared well through the last three gold rushes.
To give some perspective on the value of an original case-I have bought several 1859 movements in the $100-150 range, and paid $600 for the silver cased example shown here.
I'll update this thread as soon as I'm able to get my 1862 in hand.