Encoder knob, Akai.  This replacement data wheel fits onto a knurled shaft, is 36mm in diameter,  and is Akai part number SK-362353.
If this part is unavailable, part #1701 is a slightly smaller but workable substitute.
This item is USED.
                                    Out of stock.
                                
                                                            
                            
                                                            
                                                        $12.95
(Part #5108)
Click on photo to enlarge
