318 Main bearing part numbers versus year application

Why NOS? Clevite MS1344P are tri-metal bearings just like the originals and readily available. Also I think there was a change in 74 when the 340 went away. Not sure what actually changed however. MS540P are listed for the earlier engines. Clevite was an OEM supplier
Thank you all for your replies.
C130 Chief, I don't trust modern aftermarket automotive parts.