Inliners International Forums General Discussion Engines Conflicting information on casting numbers...

I'm getting conflicting information from the casting numbers listings linked off of the inliners site. The main casting numbers list on the site lists the the block number 3837012 as being a 1955-57 261 block. One of the other sites lists it as being a 55-59 261 application...cross generational? i thought 58 was the cutoff for the second and third gen? yet another site lists this same block number being a 1942-1951 chevy car 216! Which is correct?
Anyone have a master parts book they can confirm any one of these to be true?

Thanks!