Charlie Yapp...Google search Secrets of Speed and look under his parts section of his website.
Stipe Machine makes a nice undersize one. Google search Speciality Motor Cams.
One piece steel construction. Reduced 2-3/4" diameter slows water pump by 30%. Still uses stock Model A fan belt. Comes with bolt for attaching. Ratchet nut can't be used. U.S.A.
Part NumberSpecificsPrice A-6312-R1928-34 $99.00 / ea.
Stipe or Yapp supply the one shown from Snyder's...I just can't remember which one!