Yep, you nailed it- - - -BUT- - - -the brand new milk can lifters we put in John's 261 when we assembled it were perfectly flat on the bottom (I checked). I'm not sure about stovebolt hydraulic lifters. The cam lobes on almost all engines contact the bottom of the lifter slightly off center so the lifter spins and equalizes the wear on the cam lobe and the lifter. Putting a slight radius on the lifter helps the spin action happen if a lifter happens to be designed that way. Yes, the pushrod spins because it's following the motion of the lifter.

I'm building a fixture for my lathe to allow me to use an electric die grinder and a Roloc grinding disc to resurface lifter bottoms and put the radius back into one that's worn flat. With the length of the lathe ways I have currently, I'll only be able to put about a 48" radius onto the lifter bottom. I do have an extra set of ways available that will double that length once I get them grafted together.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!