exhaust down pipes debate

Now oners of early twin from 1960 on in which year did the reduced 1.3/8ths downpipes come on Norton twin As all early twin were 1.5/8ths full bore even the Atlas had full bore pipes So what year does the Reduced pipes come in Answers Please

The 88 and 99 sports specials could be ordered with small bore pipes.

My 1965 Atlas came with 15/8 pipes reducing to 1 3/8 a couple of inches past the ex port.
I think the siamese pipes were the same, so there is no hard and fast rule.