I was able to catch another Callisto transit on 7/13/21 (Monday night, Tuesday morning.) Even though I knew this one was coming, it still looked like a shadow transit against the white south tropical zone. Actually it was transiting on top of a long dark streamer preceding the GRS in this zone. However, Callisto's albedo is so low that it looks almost black. I am tempted to call it a lump of coal, but in the better moments of seeing at high magnification it had more of mudball look against the bright white surrounding.
After Jupiter rose high enough the air was finally steady enough on the mountain that I was able to employ 357x effectively with the 20". I could tell because white ovals began appearing, and detail around the GRS and even the NPR region began to sharpen. I even bumped up to 500x...but this sorely angered the weather gods, and Jupiter was almost immediately obscured by thin cloud/smoke that eventually shut me down. So I went back to 357x to catch it whenever the cloud thinned.
An image posted this morning gives an idea of what I was seeing:
Moonwatching ferret's: The GRS after it made it around
In the latter one you can also see the ovals in the NPR and NNTB regions I was detecting, along with the long dark barge/line up there and some subtle structure involved. I was hoping to see more of this at 500x... Color on the planet was quite good, a hallmark of good seeing. The red spot is more of orange/salmon color again, but at least it had some good color vs. what poor seeing does to it.
Earlier, before the GRS came around while Jupiter was still low, I was seeing an interesting pattern of ovals around oval BA as in the following lgwong image. That was my initial indication that the seeing was good.