The tip barrel is required to be able to continue using tips of the series TIP PT... ET... (one-part tips) with the current versions of the soldering irons LR-21 or TCP.
For current soldering irons (LR-21, TCP) Weller has 2-part tips with a smaller diameter. Therefore the hole diameter of the current sleeves is also smaller, so that the ''old'' tips can no longer be used.