Thats a Jock Kerr. The worst exhaust and parts manufacturer ever, Made loads of noise and buggered your bikes performance. The only saving grace was the comedy factor of your mates swerving all over the road behind you when the baffle was fired down the road when the securing bolt inevitabley fell out. Throw it in the back of the garage as a 70/80s curiosity. I had them on a Guzzi V50 and a Morini 350 many years ago. They were pretty common on Kawasakis in the Midlands, I'll see if I've got any pics kicking about.
They also offered a nylon "blank" piston kit to turn your 250 into a 125
Another example of their exemplary products (found on the web) I'm sure a google will turn up many more :-
"Jock KERR ran a tuning outfit in Hertfordshire (near London, UK), in the
70's/80's, they did a sleeved down conversion for the 400/4 that took it to
250cc. The head was untouched, so the combustion chamber was mushroom
shaped, VERY inefficient. It allowed learner riders to ride the 400/4,
(limit was 250).
I had one, flat on the tank, 11,000rpm, in 4th or 5th, (can't remember
which!), I got nearly 70mph out of her!! :-)
Oh to be 17 again!!"