Wilbur,
According to my data, your S/N 20,191 N is a mid 1968 (S/Ns for 1968 run 7,501 - 35,125)
I do not know what the suffix letter stands for. I have a 1970 and my suffix is R (manual start). I also have a second machine S/N 21,884 N (electric start). If anyone on the forum has info...