From what I understand the U-boats had a manually operated breechblock, therefore the sound of the empty case would be more likely to be heard as it "fell" from the breech rather than getting thrown out automatically like the FlaK 36: (9 seconds in has a good shot of the breech action)
I have just sampled a FlaK 36 as high-quality audio of the SK C/35 88 isn't very common at all.
Hope this clears things up a bit.
CD
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