John, Thanks for the info. The casting numbers on the head are 192002B and 100042A.  The casting numbers on the block are 190320H and 1K100E. I also noticed that there is a date stamp on the head which reads 2 1958 however the date on the frame indicated a 1955 model.  The serial number of the tractor is 26 324-801. The exhaust valves do not have rotators on them.  I appreciate all your help.