D-shaped ring, please help ID. WW2 or other timeperiod?

[panic]

Dear forum members, once again I am in need of your help.

Yesterday I found this D-shaped ring in the shalow water inside a tidal pool on the beach just before my hunting spot (a German-Italian sea bunker complex is on this beach and another german bunker is on top of a hill overseeing this beach).



Initially I thought the ring was rusted iron but that would not have lasted in seawater. It's non magnetic and it looks like sea-worn brass, it's redish with some verdigris spots as you can see in the pics. There are rust spots on it, too. (in the pics to help it stay upright it's leaning against the bottom part of a spent german shell I found in another pool :-)
Size is 3,1x1,4 cm (or 1 1/4x1/2 inch).
The peculiar thing (to me...) is that the straight part (that goes through a broken off partial loop thing) is round in cross section, while the semicircular part is square in cross section.

I'm no gun expert but this looks like the ring on a riffle, where the shoulder strap is attached...
Am I correct assuming this?
And on a 2nd level, is this a WW2 item in your oppinion or no relation to that time period?

[BlackCat1982]

The ring part looks too angular for a sling, the leather would might be cut by the wearing of the angled edges over it.

[panic]

Oh... that makes sense... What might this be then?

[panic]

Canteen ring maybe?