double ditto on the tuneable Dunstals.
Ran them for decades and it did make a big difference to the power characteristics of the bike.
To a large extent they got replaced with the rat traps which were really popular with the in line 4 riders.
The replicas did not have a nut in the middle tube but you could weld one into it, but the baffle usually gave up in a few years.
[Linked Image]
This is a stainless replica made by Overlander Equipment downhere in OZ fitted to the 71 A 65L
Looks like it ran best with 3 outlets open

Bike Beesa