I'm wondering if anyone can help clarify the early British 20-mm AP situation.

Was there ever a distinct Mk.I Armour-Piercing round? It seems to me there were various interim types -- but nothing that solidified into a true Mk.I.

From what I gather there were four early types that never got an official 'mark':
  1. original 'failed' design (based on French AP?)
  2. one-piece design, tungsten steel, hardened only at the tip
  3. two-piece design, mild steel body and oil-hardened tungsten steel tip
  4. two-piece design, chrome-molybdenum body and bakelite cap

The final round described above (#4) was developed into the Mk.II, while designs #2 and #3 were produced as stop-gap/interim types. Did anything solidify at some point to the Mk.I? Or was the situation confused enough that they just skipped it and went to the Mk.II ...

Thanks for any help.

Side note -- would anyone happen to have a cutaway photo/diagram of the Mk.IV AP and/or Mk.I AP/T round?

Thanks again