These are plastic plugs that fit into 1″ x .050 wall aluminum tubing to form extensions for your tripod. Generally used in pairs, one with a threaded stud and the other with a threaded insert. Tubing can be obtained from any industrial metal supplier, local or online retailer in lengths cut to your requirements. Available in 3/8-16 and 1/4-20 threads. 3/6-16 is typically used for lightweight tripods and heads; 1/4-20 fits the threaded sockets on most cameras. Please specify which you require, most people will need a set consisting of a stud and a matching threaded insert.