From what I could gather the 26 is a less expensive version of the 28.Looks like it came as a TO1,TO3,and TO4.
in Diana numbers it's an 800 fps rifle same as the 28. medium power. that's what I like.
The 27 is an older model that went out of production in the early 1980's. They have leather piston seals and no safety. They are simple, durable, nice-handling guns and great favorites with collectors.
The 26 is also very nice, but a completely different gun. It is about the same size and power as the 27, but that's it.