Shallow Well Pumps like the Model 1900 Stainless Steel Pitcher Pump are subject to freezing because the piston and foot valve are both located in the body of the pump which is above ground and exposed to the weather. With a normal piston / foot valve system, water is trapped between the foot valve and the piston. The only way to remove that water is by removing the piston assembly and draining the pump.
Bison Pumps has developed the Drain Back Feature for customers who need to winterize their pump that is exposed to freezing conditions. For example, the Drain Back Feature would be needed in a rustic cabin without electricity that is seasonally inhabited. See Drain Back Feature Basics for details on how this system operates.
This feature has components inside the pump body that the standard Model 1900 pump does not have. Therefore, it must be included at the time of purchase. It cannot be added to the pump at a later date.