Where is the gun to use Howitzer Shell (230MM) ?

[Soviet Balais]

Hello,
I am on chapter 4, I have to pound the German lines with the artillery gun and the 230mm shell, but I don't understand where the gun is..
I have already used the 4 guns gathered at the top left of the supply trench with the 75mm shells and now they are no longer usable.
Can you tell me where is the gun to use Howitzer Shell (230MM) please?
Because I can't understand where it is.
Thank you.

[umbra_witcher]

West side of the mess hall past the helmet key door

[Soviet Balais]

I forgot about this place, on in support trench map !
Thanks!