The block with the oil passageway front to rear through the lifter bores is set up for hydraulic lifters. It can be run either way, solid or hydraulic, while the block without the oil passage is solid lifter only- - - -no way to supply the oil that keeps the lifters pumped up.
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!