The 261 we just finished building for John has a 1959 casting date, and it was machined for the later style main bearings- - - -but it's the high water pump design that was discontinued as a production engine in mid-1955. No stampings at the distributor pad, also, so it must be a replacement block for 54-55 engines, IMHO.
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!