Ahh hmm, it seems like the twin barrel ones VSF has done more custom, the single barrel, I have the deco plate off one right here, it's a piece of crap:
Looks like the 8500 is custom decorated a2824/7750 and the 88xx are 2824s w/ a simple deco plate:
Reposted to its' own thread by request of k4jun. TLDR: Photo gallery with short explanations of each: https://imgur.com/a/qNBmCJC 1- The Nekton has a decorative plate that can be removed or left off with the movement still cased in the watch, which is a vast improvement from the previous...
forum.replica-watch.info
I am not really sure what the 8400 is, I'm guessing more in line with the 8500 since the difference is just the date afaik, but not sure. I want to say you can still fit a eta 2824/sw200 in there BTW, it just might require a custom movement holder you could 3d print easily (theres even project files you can use as a base). Unless the 2824/sw200 is too big or tall, that's another story. Maybe hands need special stem, but good watchmaker can sort that anyways, can't imagine any difficulty with the dial.
But yeah, not as easy as the 88xx which I had kat swap to a sw200, and that was just a drop in swap.