The spring's specific outside diameter is machined into the cylinder head to create the valve's spring seat. This is usually prevalent in two closely related engines: the 1967-92/93 A-engine (small block) and the 1992/93-2003 Magnum (small block). The A-engine springs have a 1.50″ OD while the Magnum uses 1.42″ OD.

In my brief search I could only find retainers with 10 degree locks for that size spring and stem.