okay,seems it just prooves my theory...
In fact I had two main ideas:
-one was the balance pipes would have evolved,from index one to index 2 or more,and the correct ones would have bee, the later version.
it means a derwent could contain at least 9 correct pipes in the best case,but ould countain none of them...
-other was simplier,the "wrong" were just the male pipes,of course it means EACH derwent countains 9 correct pipes.
Seems it's the second solution,would mean there are always 9 correct pipes in a derwent engine,except if someone already took them ,lol.