closed port or regular: if you have closed port cylinder 45mm it will have +++++ in a row on cylinder behind chain side cover. they are hard to find due to no real part number sequence of saws used on. the regular piston and ring can be found just about anywhere 46mm
Its a 46mm bore piston and in my little bit of research the Husky 55 and 55 rancher have a few different OEM parts numbers (EPA or non EPA im assuming).
You just need to measure the bore of what you have.
If the cylinder is still on the saw pull the muffler and get a piece of solder (make sure it is perfectly straight). With the piston at DBC slide the solder in the cylinder and rotate the piston up so it leaves a mark on the solder and measure the piece of solder with a micrometer or such.