I've seen several sets of lifters like that. Look closely at the lifter with the shallow groove and you'll see it isn't "solid" at all- - - -there's a snap ring holding the pushrod seat in, and the lifter body is hollow. Either one will work just as well as the other. It' simpler these days to use one lifter body for both "solid" and hydraulic lifters, by just elininating the hydraulic internal parts and running one outer part for both types.
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!