Surely it isn't a tank (it would make no sense to have a small, 5 or 10 liters or so tank), but it is also not a hammer arrestor, as it is not vertical, and there is no way it could provide the air cushion needed.

Very likely it is simply a sort of distribution manifold.

It might not always have been horizontal.

Oww, come on, there is no way it could have been vertical, and even if it was, it would be way too short, given the diameter, to allow for a meaningful air cushion.

Besides, in actual hammer arrestors both inlet and outlets would have been at the very bottom.

Fair. Then what's the point of that tank?

As I see it, it is not a tank, but rather a manifold to support the two valves/taps.

An example of a "modern" one (similar ones in plumbing are still hand made on the spot with pipes, but this is a pre-fabricated one):

https://media-www.canadiantire.ca/product/fixing/plumbing/ro...

The proportions are weird, which is what made me wonder. But sure.