I found these drawings in “A survey of German High Velocity and Rocket Assisted Projectiles”. The original drawings are quite small, all of them appearing on a single fold out page in a larger volume.
Figure 1 has no illustration, saying only; "High pressure. 21cm K12 Railway gun (working pressure 4500Kgm/cm2 (28,5 tons/inch2) MV 5330 fps
Figure 2 has no illustration, saying only; “Longer barrel. HDP Articulated gun. A long range gun with barrel 150m long. This gun had booster stations every 10m along the barrel so that the pressure curve became fairly flat. Projectile used comes under the heading of No 10”
If these drawings are well known, please let me know and I’ll take them down.
Happy collecting, Peter