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I can't judge the size too well, or even specifically what part is the question. But I'm seeing something that looks like it may be some kind of central oiler?
I agree with @bucket , look to see where lines go. Looks like an Autoluber for grease fittings. We tried them on a few trucks at work. Pain to get set up as each grease line pressure needs to be adjusted so that item is lubed correctly.