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My understanding is that it was related to how the fleet was originally financed by BR - the odd numbers were financed differently to the even numbers.
The first ones on the mainline were at Purley Oaks and (I think) Purley, a little over 10 years ago. Selhurst to Streatham Common were done a few years later. I worked on stations around that time and they made a huge difference.
In the rules for EC4T charges for units without meters there are factors for this. Can't remember them off the top of my head but it's something like 1.8 times a single unit for two units in multiple.
No regen braking on the 365s - were designed to be capable of it but it's never been used. Believe they will have a rheostatic brake though. Retractioned LSER Class 465s did get regen fitted.