OP. I just did a quick check on the Mossy web site. I looked at just the 590A1 models with GRS or XS sighting systems. It looks like both solid and the lightened bases are used. If you believe the pictures of the various models then you should be able to determine what model gets what base.
If you are basing your purchase on this particular feature then I would call Mossberg to confirm. It may be that this is a supply chain issue and they install whatever base is in stock.
Good luck and keep us posted.
Figured it out. The "skeletonized" front sight with the white accent is used with an XS sight ghost ring / pic rail and the solid front sight with the orange accent is used with mossberg's own rear ghost ring sight