[Mr70]

That's a Small block Waterpump pulley,intended for 1969-77 Hydraulic liftered engines w/o Air Conditioning.It was first issued in 1975.

It replaced Waterpump pulley #3989305,which was first issued in 1971.

These both replaced the original Assembly line WaterPump pulley,part #3932402,issued in 1969.
All for the same engine application as previously stated.