The model of a watch movement is the overall design of the movement. Although there may be some variations of specific models from some manufacturers, the model defines the layout of the (gear) train which usually determines the size and shape of the plates and/or bridges. The model design also includes the design of the vast majority of the parts. Some models were of less-than-great design for one reason or another, and they were only in production for a few years. Other models were the result of very successful designs and may have continued in production for more than twenty or thirty years. Waltham's model 1883 is one of those. It hit the market in 1883 and was still in production at least as late as 1917.

There are different designs of movements. The 1916 Hamilton Time Book has a variety of designs to look at:

Full Plate Model
Page 06 shows the Full Plate Model, Grade No. 940. The full top plate design has a plate that completely covers the gear train (watch people drop the word "gear" and just call it the train). There is a separate partial plate, or bridge, that provides access to the mainspring barrel (the cylindrical tub that holds the mainspring) This is referred to as the barrel bridge. The balance resides above the top plate.

Bridge Model
Page 08 shows a Bridge Model, Grade No. 950. In this design, there isn't a top plate, but only portions of a top plate, which are referred to as bridges, that hold the upper pivots of the train. In this design, the balance (wheel) is tucked in with the train, between that pillar plate (the bottom plate) and the bridges. The bridge model also has a barrel bridge.

3/4-Plate Model
Page 05 shows a 3/4-Plate Model, Grade No. 992. Its called this because the train bridge and the barrel bridge cover three-quarters of the train. In this design too, the balance (wheel) is tucked in with the train, between that pillar plate (the bottom plate) and the train and barrel bridges.

In all three designs, the upper pivot of the balance (wheel) is held by a bridge called the balance cock.