New are these screwdriver bit extensions adjustable in 50 mm increments. The bit chuck is a quick-change design in which the bit is mechanically held. This is especially important in places where there is no easy access and the bit must not come out of the extension or hang up on another screw. It is designed for screw or drill bits with a the standard 6-sided 6.35 mm shaft. The schaft of the extension is itself also a six-sided shaft of the same dimensions and so can be used with any standard driver or hand drill. Manufactured by Star-M in Japan.
Tip: if you want to use this extension for drilling: This extension is not at all designed to be used for drilling deep holes, but for use in places that would be difficult to reach. The chuck to hold the bit is 20 mm in diameter, and it would be difficult to find a drill bit with both a cutting head of at least 21 mm in diameter and a 6.35 six-sided shaft.
It is simple and quick to take out or put back the extension, you just just turn the housing on the chuck about 4/5 of a turn. This leaves the extension free to insert or remove. To lock the bit in place you just let the extension turn back to the next notch.