Open or Suregrip?

I had an 04 F250 Super Duty. It had a Sterling 10.5" limited slip. It would fail the spin in park test badly, yet smoke both rear tires almost every time. That Sterling limited slip is the same type that might be in yours. No center preload spring pack. That's why it will fail the spin test in park. There is nothing engaging the clutches. If you remove the rear cover and there is no spring pack, that doesn't necessarily mean it's not a sure grip. You'll need to remove the spider gear shaft and gears and side gear to see the clutches if it has them. You MIGHT be able to look into the area I have circled and see the clutch pack. You can see it in the picture. The blue tang that inserts into the case holds them in place. If all that is there, it's a sure grip. I'm like you. I have never seen an open rear spin both tires as positively as in your pic. But, there's a first time for everything. Lastly, because it does not have a preload pack, even a new one of this style may not spin both tires all of the time. The one in my truck did not even when it was new.