There are basically three different models. The 500, 590 Special Purpose, and the 590A1. They all have the same internals.
The 500 has a closed ended 5 or 8 shot mag. tube, plastic trigger guard, plastic safety, and a thin walled blue or matte 18.5" or 20" barrel.
The 590SP has a capped 8 shot mag. tube, comes in only a 20" thin walled barrel version, and has the same finish as the 500.
The 590A1 has a metal trigger guard and safety, capped 5 or 8 shot mag. tube, and a 14" (if you can find one), 18.5" or 20" parkerized heavy walled barrel.
For whatever reason, there seem to be two exceptions to the metal trigger guard and heavy walled barrel with the 590SP, and the Blackwater has an 18.5" barrel but still has a 590 type mag. tube.
Barrels are not interchangeable between the 500 and 590.
There you go, clear as mud?