Help Identify This Strange Mystery Intake Manifold

I was alerted to this strange intake manifold by our friend the auto designer John Frye, who spotted it on the Innovative, Banned, and Unique Racing Vehicles Facebook group. I see why it caught his eye: it’s a richly strange and puzzling manifold design, and I can’t think of what engine it might have been designed for. I suspect one of you loons may have an idea, so it seems like a good idea to put it out there.

Here’s the post from the group:

It seems to be a triple-carb manifold to feed a six-cylinder engine—a V6? I suppose it could be a straight-six, too? It looks like it’s basically three stacks of flattened intake runners, in funny U or wavy shapes, each fed by a carb.

There’s also this number stamped on the manifold:

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H-403-A? I think? Does that ring a bell for anyone? 403 could refer to an Oldsmobile 403 engine, but that was a V8, so I don’t think that’s it.

It’s a really odd design, so I’m quite curious, but also quite stumped.

So, one of you greasy-handed car dorks out there must have some idea, right?

Let’s see if our big, wet, pulsating collective brains can help out! Gimme your ideas and theories below!