Chevrolet made subtle changes to points through the years. some are two piece, some have a solid contact surface, some have a hole in the center of the contact surface, etc... Any time there was any change in the part Chev gave it a new part number, although they will all interchange as a set.
As for the polarity reversing switch. In an attempt to make points last longer Chev added a switch to the starter linkage which switched the direction the current flowed through the points every time you starter the engine. This was an attempt to keep metal transfer from one contact to the other.
The distributor also needed to be different, so the points plate could be insulated from ground. The reversing switch decided which side of the points would be grounded.
Of course the switch ended up to be more of a problem then changing points once in a while and was discontinued.
