Hi Caius
First great post and photos !!! The Chronostops was a very popular model for Omega and the most important part was they were USED !! Your comment "....but most are in bad condition!" is in line which what I have observed over the years. I have seen them used "HARD and put to bed wet !!" a true tool watch.
The 865 as you stated is based on the 861. You have a great movement in a rugged steel Omega case.. a great combo!
Havev a great weekend
BIll Sohne