The regs on emission controls you refer to came in as CK says at end of 1972.
In them days I am pretty sure it was 91dB but things like decibel meters were too expensive for an mot tester to buy so they usually just checked the pipes had their baffles fitted and wernt cut about too bad. We used to buy pipes called dB90's which were made by Dunstall Performance. They supposedly increased power and were just road legal.
http://www.all-digital.net/gftp/muff.htm
Because decibel scales are not linear but logarithmic, 90dB sounds a hell of a lot louder than 81dB.