The retro fit continental kit for the '51 Kaiser was announced January 22, 1952. The bulletin states that "the use of a Continental Spare Wheel Mount on the new 1952 Kaiser models has created a demand for this unit for installation on 1951 Kaiser models..." so it could have been retro-fitted on a '51 but as Gordie points out, if it's a '51 it would not have shipped that way.