The offset-hole heads were used from 55 through 59, and there were some aftermarket cast aluminum covers made by various manufacturers such as Offenhauser, etc. There are also some very pricey original-equipment Corvette script cast aluminum covers still around. It used to be hard to give away a set of offset-hole covers - - - -now they're a hot item for original-condition restorers!
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!