Hello, I must correct myself:
the casing "L1111" was (according to Stegmueller) probably used in 14/10 experiments in conjunction with a conical barrel proposed by Polte as an improvement for (or over) the PzB 39.
A casing of the same dimensions (as far as can be judged from the drawings), as part of a round described as "Patrone 131/8-14/9", was used in tests with an altered MG 131 barrel. It is the latter which is shown by BLU 97 and which was part of the "Goering" ruse.
Involvement of Herr Viereck in both experiments and misreading the German text had me confused. I apologize!
But, to make things even more interesting: there also existed an unbelted 14/9 round, this was part of an experiment with the Patrone 318 (PzB 38 and 39) as ammo for a machinegun to be used in tanks, the MG 141 EW (I do not know what EW stands for).
In this project a conical barrel was also tested, this was the unbelted 14/9 (I presume the casing was based on that of the Patronenhuelse 318, Stegmueller does not specifically state this, but overall shape of the case, neck excepted, resembles the 7,92 x 94).
Regards,
Menno