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I've asked around and no-one that I currently work with seems to know. Perhaps it's just a standard part number and there is no answer? I know this isn't a very important topic, so feel free to delete this post if it's a bit irrelevant. Thanks.
Hi Brian, I get notifications when someone posts a question, so I thought that I'd jump in right away. After a bit of research I think that I've found a possible answer.
You're referring to buttweld pipe fittings, and I think that the wpb stands for 'Wrought Pipe Bend'.
The steel for these fittings is is wrought rather than cast or forged. When steel is cast, it is heated until molten, then poured into a mould where it cools and solidifies. Forged steel is cast in the same way, but then heated to a high temperature when in a solid state, and suddenly cooled. This is how swords are made, the steel gets really hard.
Wrought steel has two processes applied on it:
This article has more information about the different processes of commercial carbon steel.
Since these pipes are wrought, pipe shaped, and bent, I reckon that this is the right answer. Could it be a coincidence that a random part abbreviation also fits? If you know the answer please comment below!