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Here are some pictures of an original Petroleumbüchse für Ruckstoßverstärker for the M.G.34. No markings on the top of the lid, only manufacturers markings at the base. Manufactured by Schneiderwerke GmbH (bdk code) in 1943.
Inside the can are two round sheets of stiff cardboard (Preßstoff?). The one with the larger diameter seals the lid and the other fits into the base. I assume these round plates are original for the can as with them inside the Mg34 recoil booster assembly fits perfectly inside.
7,92lover

Yes, the disks (sometimes leather) are original to the soaking jar and prevent the muzzle booster from rattling.

These were made for the MG08, MG34, MG13, MG15, and MG42, and for blank firing adapters where available, in sizes appropriate to the purpose.

Thanks for your reply.

7,92lover