Yes it a differential gear, with its crown wheel. it's definitely not limited slip so,it's worth very little, you could count the teeth, and research with a certain matching pinion gear, it's turning ratio, that might be a start, but if it's was used, and you had no idea of its model, it would be worthless, unless you were reading a newspaper in a gale.
Maybe a conversation piece, it's a very clever piece of car technology etc.

Don't know it's size but could it be from a tractor?

could it be this,

Click image to enlarge

Thanks for all the help, I'm still unsure as to what it is exactly for so I've donated it to someone who may make use of it, shame to bin it.

I think Stan is right....