I do believe it is for line splicing as mentioned. If there is a hole on the end of it, push a small screwdriver or something similar into it and feel for a 'springy' feel.
Well I don't know quite what to call it but I can tell you that I have dug at least three of these (all different sizes) and all were near utility poles. Those I've dug also have copper wire sticking out of each end.
I would have to agree with some sort of inline splice/connector that was cut out and either dropped or discarded.
They're solid copper and pretty hefty so I can't see just tossing such an item.
Definitely a Splice. Auto type, no crimp necessary. There should be a number on it somewhere that would let you know the size of conductor that slips into it.