As was said, some parts may have been changed out, that's one of the beauties of this saw series. If you'll remove the top cover and look at the inboard throttle lever, a small wire toggle between carb and lever would be consistent w/ a 181 carb configuration. A plastic lever only, pushing down on the carb throttle rod cam is 281/288. The 181s had the interconnecting wires between ignition coil and module molded in (pain). Later styles had friction terminals. The only way to know the top end for sure is to pull the cylinder. 52mm is 181/281. 54mm is 288. The 181 has a much smaller combustion chamber, which really helps it's performance. However, it also has a drawback. The exhaust port is smaller than the 281, and the intake, exhaust and transfers are smaller than late style 281/288s (w/compression release). Long story short, a 181 is a real performer with a little port work. Don't scrap your muffler, just straighten it out a bit and do a shotgun mod as seen elsewhere on the site. Make sure your bearings are good and let'er rip. And double check the throttle linkage, whatever type, is moving the carb to WOT. This is a common issue on both linkage styles.
