Guess what this is

[Tony Press]

I too think this is a later iteration. The example shown was in use in the sixties on small round sealed cans of oil, as refillable bottles were being phased out.
I have one from the  teens and twenties, it is a brass and steel stirrup that lodges under the corner of the 4 gallon square tin can. There is a handle which presses the spout into the top corner. These tin cans came as a pair in a wooden box for delivery into parts of the country where there was no pumped fuel supply.

I think Russell is right - that is a sixties oil pourer for oil tins before plastic took over. Not sure they were around in the twenties or thirties in that form. 
Mind you I haven't seen the square tin pourer .