Another memory from pre-history just surfaced. Long ago 16mm instrument-camera film would often be chucked in a bucket for development etc. All you had to get off it was the time (from the clock in the shot) that the other dials and numbers showed whatever-it-was. So, as the OP jokingly suggested, buckets really have been used to develop long bits of film - though with some slight damage that probably made the results unusable for enlarging.