I can only see this on my phone at the moment so might be mistaken but it doesn't look like a S&L cross. More likely a Meybauer.
Looking for LDO marked EK2s and items relating to U-406.....
Man, you are good. I pulled my marked '7' out to check and darned if it doesn't match ever so closely. The '3' in 1813 was throwing me a curve. Thank you, Sir!