They are very nearly the same, and will work interchangeably, but... There is a difference I'm aware of in the method of locating the main bearing shells. The early ones weren't really located, other than by the bolts. The later ones fit in a closely milled channel...a big improvement that is rarely mentioned. This doesn't really mean much, since the shells have to be kept in the location where they were machined and can't be swapped between engines.
That's the only difference I know. Maybe others know know of more?