There are four variations each worth discussing, see below for the links to detailed pages.
If you are impatient and want a quick precis then go here. Then you will probably come back and go through the rest of the site carefully.
This is usually the reference people look at first, and with good reason. It has the right combination of age, historical association, appearance, ease of service, availability and value. Even within this reference there are interesting variations. There are three variations of case back, and two bezel styles. The dials can age particularly well, with some turning a lovely galaxy chocolate brown. We see this most often in the 2911xxx and 2960xxx serials.
This reference is relatively plentiful, and so with care you can find a nice one. Things to check as ever are the correct hand layout, correct dial and the case condition. With this reference we can afford to be selective. There is no reason to accept a worn case unless it is very cheap.
These are sometimes fitted with service dials, and a quick note on 861 service dials is here.
There was a commemorative 18Kt yellow version, and there is a fantastic article here
There are four sub divisions of this reference, dealt with on separate pages via the link above or click on the images below: