From the book "Chevrolet V-Chevrolet V-8 Engine Casting Numbers 1955-1993":
Although the basic design of the Chevrolet V-8 water pump has remained the same over the years, many changes and alterations have taken place. Certain models have bypass hose holes or mounting bosses for engine accessories. Heater hose hole locations change from model to model as well. In 1969 both big- and small-block water pumps were changed fram a "small" to a "long" design, to accommodate Chevrolet's new configuration for mounting engine accessories. The 1969 casting was altered, leaving noticeably more distance between the back of the water pump and the timing chain cover..."