Help Identifing Part Sears ST16 Tecumseh

Can anyone tell what this group of parts does??? And what it is called. Also what can I ask for it or expect on ebay. I am selling several parts and some, I am unsure of value or approximate value. Appreciate your help. Thanks, Buzz

For engaging and dis-engaging the mower.Not sure if there is a technical term for it.Price really not sure,imagine would not be too much-usually the decks rusted away but that piece was always left on the tractors.

As per the first response this is the belt tightener for the mower and possibly other attachements.... point missed is that it is also one of four (4) deck hangers on which the deck mounts... without all four you cannot hang the deck. Value is basically if you need this part, but my point is you need all four to hang the deck.