Yes there was such a thing as a 270 degree roll,but usually this was a 3 out of 4 point roll,which is then held at the 270 degree point.Providing the entry speed is fine at this stage,a one and a half knife edge flick is then initiated,finishing on the opposite knife edge,which is then followed by the remaining quarter point roll back to the horizontal straight and level.This should be completed from straight and level flight.Back in the 70s Rothmans Solo man,usually Iain Weston,used to perform this frequently,as the Pitts has the power to weight to achieve it without losing the necessary entry speed after holding the third point in knife edge.Bucker Jungmeisters were capable of doing this as well because of exceptional control surfaces,with only 160 HP available.Another Jungmeister owner and pilot,the late James Gilbert showed me how to do this with relevant entry speeds at Redhill back in 73.The only point of achieving a 270 degree roll would be to end up on knife edge,either to then follow with the flick or proceed down the crowd line in knife edge flight for the purpose of showing the Aircraft to the public/photographers.A Derry turn has nothing to do with this,as explained by the posters above.Hope this is useful!!??