Complete replacement front end assembly.
Will fit Chinook Pro 1 booms and many others, including quite a few Aeron, Naish, Pro Limit, RRD booms for example.
Please make sure that your boom matches the pivot (the C shaped connection on the rear of the mast cup T Joint #219T), which has an internal diameter of approx 32.5mm and a length of approx 87.5mm and fits inside the Shell #219S to encase the original front tube pivot point for the head assembly on your boom. (See photo's)