I think you are referring to some of the very first Gen1 guns, based on your comment I called Glock and the technician said zero internal differences between any Gen2 and Gen4 gun, grip frame differences are the only differences.
The Gen1's were the ones that had the 6 part upgrade. Frame rails...there were 3 different frame rails on the Gen1's, and the last style on Gen2's and later; so you are correct.
Apologies, my memory failed me; I thought the 6 part upgrade came with the Gen2's, but it was Gen1's.
http://reducerecoil.com/2015/03/difference-between-glock-gen1-glock-gen2-glock-gen3-glock-gen4/ 
