Help with ID of German 13mm cartridge

[roadrunner404]

Good Afternoon All, was wondering if anyone can help with identifying this German cartridge for me & what it was used in please. Though it might be experimental but not sure.
Head stamp details are: P at 12o/c (Polte?); 39 at 3o/c (year of manufacture?); Waffenamt (WaA406) at 6o/c & 21 at 9o/c (lot/batch no?).
It appears to be made of brass & has a two-hole primer pocket.
Dimensions are:
Calibre: Appears to be 13mm but the casemouth is distorted slightly.
Overall length: 94mm
Rim Dia: 20.9mm
Groove Dia: 17.2mm
Rim Thickness: 1.85mm
Distance from base to bottleneck shoulder: 69mm
Length of bottleneck area: 13.6mm

Have tried to research this one but have had no luck in finding any results; many thanks for any advice or ideas in advance. Regards, Ian.

 

[rigby]

It's a 13x94 for Training barrel of the PzB38.

 

[roadrunner404]

Many thanks for your help Rigby, much appreciated! Rgds, Ian.