The "TL" design is known in the industry as a "Dodge®" style taper lock bushing.
The "TL" taper lock bushing was originally invented by Dodge® Industrial in 1939.
TL taper lock sprockets are specialy designed for use with roller chains in power transmission systems, featuring a unique flush mount taper-lock design for easy installation and removal.
The "TL"design allows for flexibility in bore size options without having to replace the entire sprocket, by simply changing the accompanying taper-lock bushing.
The "TL" design facilitates the bushing to nest flush into its mating hub so that it maintains a much narrower profile than the competing "Browning®" and "QD" bushed sprocket assemblies whose flange and bolt heads protrude from the sprocket surface much greater.
The "TL" flush mount design does come with a caveat. The flangeless split taper lock bushings have a tendency to split into two pieces when removal or realignment is required, whereas the flanged "Browning® and "QD" designs do not suffer this weakness. Ringball Sprocket Catalog