That's an oddball lifter that's been installed sometime in the past. The only reason for the hole is an oil drain- - - -no other purpose. Be sure the lifters go back on the same cam lobe where they were running before, or the cam and lifters will wear out in a very short time. Each lifter and lobe has its own distinctive contact pattern, so mixing up lifters will result in a very short life engine as the lobes destroy themselves and spread contaminants through the whole lubricating system.
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!