Hi Mat
I think the slide rule bezel comes with a yellow'ish finish. I have 2 cossies from that period (81600B and a Scott Carpenter, neither display back though that is arriving next week) and both have similar colour bezels. I have just got my 81600 back from Breitling after an overhaul. They fitted a new bezel at that time which has the yellow/gold hue already. Interestingly I asked for the bits they replaced to be sent back to me (the AD told me they were unlikely to do so) but when the watch came back it was accompanied with all the replaced parts including all the hands, bezel, pushers and movement components in a little bag. Someone in Breitling was feeling generous as the bag also contained an extra slide rule bezel that looks brand new, it too has a yellow/gold colour. I assume the display back cossie has the same slide rule bezel as the conventional cossies from that period.
Do you think yours has changed colour since you've had it?
Don