My understanding is that the changes of speed rating which applied to all films at the time (was it the late 60's of 70's?) were the result of changes in measuring definitions by the relevant Standards Organisations (probably ASA and DIN ratings at that time). There were no actual changes in the emulsions.
Originally Posted by Dr Croubie
I think it has been said that it was to allow for the more accurate meters and cameras which were becoming available, which made the previous allowance to avoid the risk of underexposure no longer necessary..