Mystery casing

I have just acquired a new casing and I am unsure of what exactly it is. It’s a 12.7mm/50cal of some description however it is only 63mm/2.5 inches long and I can’t seem to find anything matching that description online. Also I can’t find any information on the head stamp it reads 40 19h BK there is also another marking however it is a light stamping and not complete, it does have a staked in primer. Something I thought was odd about the case was that the rim looked like it had been cut however the crimp line is intact and the shoulder is uniform like it was factory done. The case has been polished and I purchased it with a batch of other various size cases one of them had been made into a presentation piece I assume, it had D H H stamped on the side of the case and the rim has been cut down to remove the crimp mark. Apologies for the long post it’s my first time here.

Hi
It’s a 13x64 MG131 case.

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Thank you for that I had a hard time searching for it

What is “BK”?

Source : municion.org
Metall, Walz und Plattierwarenfabrik Hindrichs & Auffermann A.G., de Wüppertal.

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That source is incorrect, because the headstamp is not “bk” but “BK”, that correspond to Hugo Schneider A.G., Werk Altenburg (wg).

Regards,

Fede

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For those who may wonder: Luftwaffe originally had its own code system of uppercase letters, to be used on Luftwaffe-only calibers. For example, Polte Magdeburg was assigned BT in this system. During 1940, it was decided to switch to the Heereswaffenamt codes of lowercase letters.

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Jochem,

I am not sure it was only used as a “Luftwaffen” code.

Here an example from a Polish PZB 7,92x107.

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Willem, HASAG was making lots of air force ammunition (if not mainly), and with changing codes some manufacturers were irritated as you know. Maybe this happened with HASAG and the air force codes too?

The „Abnahme“ of the BK case showed by bubbels shout be Luft 54.

On the Polish case WaA 201, Not a Luft Abnahme.

I know but it is still HASAG.

Yes I Know, means Luftwaffe and Heer had both there own “Abnahme” officers in the same facilitie by HASAG in Altenburg.

Willem,
you are correct that Heer (Abnahmestelle), Luftwaffe (Bauaufsicht) and Marine (? not sure) had different acceptance organizations. All three showed up in Mauser Oberndorf wartime phonebook I once saw.

A notice to all departments of Polte Magdeburg, cited in the book “Zahlencode System” by Johannes Preuss clearly says regarding “Fertigungskennzeichen”:
“für sämtliche Luft-Kaliber = BT” [for all air calibers]
“für Heer und Marine = aux” [for army and navy].

I frankly have no fact based explanation for the anti-tank cartridge you show, beyond the speculation that it could have been ordered by Luftwaffe for its ground units (airfield defence etc., or even Paratroopers). Considering the mentality of the Heer in the position of an unloved child compared to how Görings Luftwaffe was pampered, I have strong doubts that army officers would have accepted Luftwaffe codes over their own. But we lack hard evidence.