Model Number: 3W Silver Plated McWilliam Model Inner Diameter: 17.7mm Throat Diameter: 4.0mm Cup Shape: U Cup, Tilz marking F, Shallow Shank: American This is a larger size than the McWilliam 3. About a year ago, I felt the 3 was too small, so I got this one, but I haven't used it much, so I'm selling it. The shank has some scratches from being attached to an instrument, but the cup is clean because it was stored in a box.