Here are 4 pcs of new old stock shaft extenders for 1/4 inch diameter shafting and controls.  Made by the famous electronic hardware supplier HH Smith, model 160, shaft extension is 5/8 inch, shaft coupler part is 1/2 inch deep, single set screw for attachment. Handy itm for homebrewers, used in WJ radios which is were I got these. These were removed from packaging for store room bin but NOS otherwise.