OTB: Later today I'll try and find my engine parts catalog to see if there is a parts number. Even if there is no number, you could still try to contact The Refactory or Classic Mercedes in southern Cal. I know the Zenith 3t/40 was used in some BMW so you could check some of those forums.

As regards disabling the fuel return valve, look at more info on that on the forum. Obviously some folks don't need it because they don't drive longish distances or time in hot enough weather to prove heat related gas issues in their carbs.

__________________
Currently enjoying:
1967 250S - all manual with 4-spd on floor-- my Nifty 250