The F stands for a full wrap handle or western type handle. Out east most of our Pioneers came with the half-wrap handle, so our saws would have the letter H after the model. If it has the safey brake then an S is added. A P61SF would have a full wrap handle and the safety brake. The P60 and P61 are pretty much identical saws. Most part numbers are the same. Same displacement. The P61 is a newer saw. The P series of saws started with the P40, 50, 60's in the early seventies. The next series were the P41,51,61's which I think were introduced around 1978. Following this came the P42,52, 62,65's and finally the Poulan pro models 405, 455, 525, 655. In between they also had the Farmsaws, FarmsawII's, P38, P39 and Farmlites. The P61's that had the safety brake also had a diecast chaincase cover that covered the muffler. The P50, 51, 52, 525, P60, 61, 62, 65, 655 are large frame saws and parts will interchange between these models. The P40, 41, 42, 405, Farmsaws, P38, P39, 455 and Farmlite are medium frame saws. Parts will usually interchange between these models, but not the large frame models.