These are great saws once you get to know them a bit. They were a huge success in 500, 5000, 5500, 540 and last 543.
Husqvarna kept the design in 50/51/55 and use same crank/piston/cylinder in various variations.
Poulan 325, Jobu 949, Jonsereds 490/590 and some others was also made.
Wonderful...