I have a 1937 18-28 Cockshutt Hart Parr.  As does yours, my tractor has Oliver on the engine plac and Cockshutt embossed on the radiator.  On the left side of the 18-28 engine block &amp;quot;Hart Parr&amp;quot; is embossed. 
Oliver purchased Hart Parr sometime around 1929 or 1930.  They began building tractors for Cockshutt I believe in 1930 and continued until 1944 although very few were sold during the war.  I believe your tractor is correct as it is.