Vintage Large socket SP-1315 specialty Socket tool

Im very unfamiliar with this large socket, It weighs 1 pound and 10 oz.  and is very well made

I thought i would try putting my 1/2'' Ratchet on it to check the opening out and for some reason my ratchet head will not go in the hole

I measured the opening at 12.5mm and the head of my Ratchet at 12.5mm but it does not go in like a normal socket would, I put on my Jewelers helmet and enlarged the view, There are no Burr's or ware, but looks like its made unusual so it may be for a special extension

If you look at the 4 sides you can see there are rounded grooves on each wall so its not made Flat inside the opening where the head of the Ratchet goes,, So  I am not familiar with describing this and do not know what it goes to

Maybe this is a Semi Tool and a Railroad tool, I personally have never had to use a Socket this big other than one that holds the Center Nut on for an Axle to a Car,, But thats as big as ive used and they did not weigh almost 2 pounds and this heavy duty

'''PLEASE LOOK AT THIS TOOL GOOD BECAUSE I DONT KNOW WHAT IT GOES TO''''