(picture shown is 3056 for reference only)
These T-nuts are sized to fit in the T-slots of the Sherline lathe crosslide, the mill table the rotary table and other accessories that have T-slots like the 90° angle plate. They are tapped to accept 10-32 screws. They are not just square nuts, but have an extended shank that fits into the table slot to maximize thread engagement of your hold-down screws.
All Sherline accessories that require T-nuts come with the proper number, but because of their small size, they are sometimes misplaced. Extra T-nuts are always handy to have around.