Watts 1 1/4" -2" 800M4. Kit includes float disc assembly, vent spring, bonnet, bonnet o-ring, and hood and screws. This kit is designed to retrofit an existing Watts 800M4 1 1/4", 1 1/2" or 2" size. The difference between a "FR" (freeze resistant) model and a regular model is the vent float that has a release to give room for water to expand and to help relieve pressure under the float and to hopefully keep it from breaking during freezing.
Of course it is always better to get the water out of the assembly for the winter but this is a measure of added protection.
It is possible to just buy the freeze resistant float as well part number 0887716.