The separate surround would be for the 280 style lights with replaceable lenses. The threaded studs on these are 40 to 50mm long whereas those on the 230/250 style surrounds are 50 to 60mm. I think the difference relates to the different bulb holders - the 230/250 versions must be 10mm thicker than the 280 versions, necessitating a longer stud to get through. Apart from the different stud lengths the surrounds appear identical and are all numbered, on the upper inner corner, 2235R (or 2235L). It looks like the threaded studs could be extracted and replaced with longer ones if necessary but you'd only need to do that if you're building repro 230/250 style lights.