The 50 was originally 49cc it has a 44mm bore the early 51's also had this and then progressed to a 45mm bore for 50.9cc at some point the 55 came out some 55 had a 53cc 46mm bore and others had 45mm bore. some 51's also had the 46mm bore. Basically the only way to know for sure is to measure the bore or look on top on the jug by where the spark plug is, it should have 44, 45 or 46 in blue or black paint, there is your answer. Power output ranged from 3.0 to 3.4hp. the port timing could easily make the difference from one to another. You could probably see that much difference from one saw to the next right off the assembly line. I have noticed that there are some differencesin port shapes in sizes between these models.
buck