Help with 3.7cm BK case drawing

[Weasel]

I have seen these drawings showing a 3.7cm H-Panzergranatpatrone L'spurUb o Zerl and Hartkern, but it is not like any other that i have, it has a very odd bottle neckand the size of the belt is completely different to the normal 6348 St case. Is this an experimental case ? or are the dimensions just really off ?
Best Weasel

 

[pzgr40]

Just a terribble drawing.

 

[Weasel]

I wondered if that was the case, but in the same publication is this drawing showing the HE BK round in correct case with a 48.6 width of belt on case, while the previous shows a width of 67.5 ?
Best Weasel.

 

[Alpini]

These 3,7 cm cases are known from other publications too. Stegermüller describes them as "most probably experimental state" with no further details known.