fergi -
Here is what little I know about these guitars (mostly courtesy of westerlyguildguitars.com):
D50 - one of the first dreads produced by Guild in 1963. Spruce top, rosewood back and sides, "X" bracing and dot fret markers. Binding all around except on fretboard. I am not sure if only the early D50's were called Bluegrass Specials, or if there is another model with slight differences.
D55 - originally made as special order only, became a regularly produced item around 1968. Spruce top, rosewook back and sides, scalloped bracing and "fancy" square fretmarkers with pyramid inlay. Some models also had additional abalone inlay around the soundhole. Binding all around including the fretboard, with double-line overlay of some sort on the fretboard, top and bottom. Special G shield logo on the headstock. So-called "flagship" dread, although there are D60's and 70's out there that some may argue are more worthy of flagship dread status.
DV52 - as the "V" indicates, this was designed to provide a "vintage" sound, primarliy through scalloped top bracing. Spruce top, rosewood back and sides, and dot inlays up the fretboard. Additional abalone inlay around the soundhole. Binding all around except the fretboard.
So all the D5X's seem to be basically the same, with largely cosmetic differences, and maybe some differences in interior bracing. I could be off as I am no expert, but this is what I can provide.