Fwiw the '81 Holdworth 'bike aids' catalogue at nkilgariff states Cinelli(?) road bars as being 40cm with no options. The GB bars were probably sixteen inches outside to outside, or even 15 7/8ths or something. I think the wider 42/44cm drop bars were starting to become an option in the late '70s, but perhaps some suppliers thought it was a passing fad/gimmick and couldn't be bothered to stock them?
I seem to remember Whisker's late '70s/early 80s price list showing 40/42/44 options for Cinelli/TTT, but when I bought a pair of 65s back then, Joe wasn't soliciting my opinion as to my optimum bar width- he just handed me a pair of 40cm. Make of that what you will.. (Actually even at the Cinelli price point, it is not an exact science- my nominal 40cm bars are actually closer to 39.5cm c-c)
Old Ned: I'm not sure if Merckx used wider bars- if he did he probably invented them, and, being Merckx- was probably in a position to get Cinelli to custom build them for him, years before they became available to the rest of the peloton, let alone the likes of you or I ?