I looked at Diesel Giant's writeup on flushing coolant, and if you scroll down to the picture of clear water being flushed through the block, you'll see the same thing that you saw. Might not be a washer. Have you looked at the EPC diagram to see if there's a washer?
https://dieselgiant.com/mercedes_citrus_flush.htm

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.