I don't know where this rumor started. Kodak's latest technical data for the T-Max films manufactured in its new plant, Publication F4016, can be found here:(posted 8122 days ago)http://www.kodak.com/country/RU/ru/professional/support/techPubs/f4016 /f4016.jhtml
It still shows 120 T-Max being on a 4.7 mil base, unchanged for at least the last dozen years. When first introduced these 120 films were on a 5 mil base, but that was reduced to 4.7 mil *very* quickly, probably not later than 1988 or 1989.