Here's a REAL quick explanation...
Seneca- Lyc IO-360s, pretty much a dog, slow and don't climb too good...
Seneca II changed to TCM Turbocharged engines. I beleive these were wimple fixed wastegate turbos. A good bit fater, and finally got some performance.
Seneca III a few changes, but basically a II. New instrument panel (one big flat piece of aluminum without the plastic overlay), also has a one piece windshield.
Seneca IV New Interior again, and asymetric engine inlets (the little circular inlets)
Seneca V New panel (switches above the windscreen), and I think this is where the new turbo controllers were put in, real simple turbo operation--put teh throttles someplace and the MAP stays there...
That's about all I know.
Dan