Forum member Graham sent me this parts diagram link
https://www.sls-hh-shop.de/main/de/mercedes-280-560sl-w107/47-tank-und-benzinpumpe/47-c-kraftstoff-versorgung-ab-09-1985
Assumption: If item 63 is the same valve as in my pic, then the leaky hose in its vicinity is item 66, p/n A1244762726 (Shaped hose to vent).
Someone with knowledge please confirm or deny my assumption. What size is that hose? I don't want to buy it from Mercedes, I want to buy a replacement hose from an auto parts store.