I dont know much about this other than it has never been used and is put together already.  Box says, whitewater pitless adapter top for deep well pump model 300.  On cap itself, it has the number W 310 6, and W 306 6.

Please know what you are looking for before buying this. I will accept returns on this if it the wrong one, but at the buyers expense.

PLEASE SEE ALL PHOTOS AS THEY ARE PART OF THE DESCRIPTION