Originally Posted by AkTom's
The only differences in the hooked end are when they were done and why. Thousands of the commercially made semiautomatic Type 56 copies were being produced by various plants and being imported by several importers, (Polytech, Norenco, etc.). Many parts and rifles were already in the pipe line when the US regulations were changed. The existing original bayonets, ("Short" version), were modified by opening the mounting hole into a hook shape and adding a set screw or pin to the hilt to keep the assembly together when removed from the rifle. After that the blades were redesigned with the "Long" version. This version with the large through pin was more complex to produce and inherently weaker and prone to breakage and loss. The large through pin is a friction fit and held in place only by spring pressure. The elongate hole weakened the mounting point and added free play to the bayonet when extended.
The various import restrictions that were changed several times over the years. This was in the American public's concern over "AK47s", "assault rifles", "current military weapons", etc.. The various restrictions were emplaced over the years to de-militarize and sporterize these rifles. The Chinese as well as other ComBloc countries were willing to make these modifications to be able to sell these rifles to the lucrative US market. They had the experience, machinery, equipment, surplus parts to mass produce these rifles.
I totally agree with your final statement! These were battle rifles and the attached folding bayonet was always there when needed. Easy to extend for use and impossible to misplace or lose. The semi-permanent attachment point, (machine screw or rivet), was serviceable if replacement or repair was required. There was no need to make them easy to remove or for the hilt and blade to be held together when remove.
After additional research I believe the change from machine screw to rivet pivots was made approximately when the change was made from AK47 to AKM style in the mid 1960s. It would make since to make this revision and simplify production during this change over. There may have been some overlap as the various arsenals used up existing parts.