Ah ! the numbers OC71 , OC72 and the Mullard circuits. That was a nice walk through the yesteryears - I still remember the audio amplifier that I made with these some 30 years back in the school days. And the way they used to get heated when driven to clipping in all eagerness to get "more" volume - and once into that used to switch them off wait till they cool and then do that again... Heat sinks ? who cared ( and knew) anyway !

Raghu