Scrubflex -
The biggest single difference that I can see between the one-piece Straight-Vane and the two piece appears to be the taper, or sweep of base fins on the two piece. Compare the top of the fins to the one-piece's fins. The one piece has a step-down notch on them, whereas the two-piece does not.
I believe the two-piece base is used on a number of agitators, including as you mentioned the WP Surgilators, and maybe even as a base for some DAs?
I remembered one model which is where I think I first noticed the two piece agitator on the Sears floor. Check parts direct for model number 110.92260100. This is the single speed, Super (aka Large) capacity 3.0 cu. ft. Kenmore 60 series washer from 1992 which had the two piece straight vane.
The parts drawing is odd as it shows two agitators, one being the top of your two-piece unit, and the other being a full single piece SV. The original part numbers don't point to a single piece SV however. This not the first time that I've seen slight inaccuracies in a Kenmore drawing, however it lists the screws you mention that hold the top and bottom parts together, as well it gives you all the original part numbers should you care to look for them on ebay.
I don't think this agitator was used very long - I didn't notice the taper of the vanes being different between the two SVs until this post, but I seem to recall the 60-series machines not selling well, then when Sears did offer a white on white model which did sell well a few years later (it sold by the truckload) they were equipped with the one-piece agitator.
Good luck!
Gordon[this post was last edited: 6/19/2011-20:17]