SP stands for Semi Pro......that's a nice looking 520. Never cared for the 490 or any of the 5XX saws. Though I must conceed the 490 was the best of the worst. It's not the SP designation that I don't like as the 49 SP was/is about my most favorite saw of all time. The 49SP, 50, 51, 52/52E, 521E, were all 49cc old school "pure" (as Niko would say) Jonsereds. The 490, 510, 520, 535 etc and whatever other variants of these saws were 49cc as well. But they came at the last gasp of the old company were very poor excuses indeed...(compared to the earlier 49 through 52 series) Of course this just my opinion......but when these saws came out Jonsereds was the saw to have in my area.......every firewood guy and pulp cutter/logger used them. A lot of 49-52s, a pile of 621s (cadillac of the day), 70Es and 80s ruled. This series of saw completely trashed the brand. All the wood cutters bought the "newest" series thinking they would be even better than the old.....much disappointment and many burnt up saws..dealers couldn't keep them running and either stopped selling the brand or went out of business altogether.
Guys around here look at my collection of Jonsereds and say "HMMMM Jonsered.... they don't make those anymore??? Do they??" They are gone from the market. Everyone divided themselve between Stihl and Husky pretty much though I believe the later offerings are great (like the 2171, 2188 etc) no one here has ever heard of them.
I blame poor performance from Tilton and these saws for this situation. Sorry you wanted opinions good and bad.....but in reality, if it runs good and you like it have fun and enjoy.
Probably a good enough saw to have and use a little but in my opinion the SP designation is even to generous.