Citizen pioneered the use of black coatings on light alloy cases in the early 1970s. I’d noticed one or two images where the coating looked to be brown. I saw one example for sale a while back which was advertised as a ‘brownie’ (Citizen called their black coated watches ‘blackies’) but I thought this may have been a result of wear. However, I recently got hold of a Cosmotron from late 1974, in good condition, and this has a dark brown finish, rather than black:
It does appear to be the original finish. I shall keep a look out for other examples…..