I have a 510, 511, 512P (gave my son-in-law the 512) & a 521T. I never stayed awake nights dreaming of a 513T, but lusted after a 513S (not in my price range as a teen or now). I respected the 550 because my uncle had one that was stone reliable & caused the demise of many squirrels. He used only shorts.
All of my .22 rf autoloading rifles are Mossbergs: four 152 & a 151K.
Recently I found a 550-1 in above average condition for $220.00. Considering what I saw on Guns International that seemed like a good price. According to the bbl stampings it was manufactured 12/49.
It's all there except the sheet metal empty deflector (the attaching screw is there).
I'll give it to our youngest son as he makes some amazing shots with the 511 that I gifted him. The 550-1 has the same sights as the 511.