So, here is what I know...
AIMPRO Tactical:
http://aimprotactical.com, has made quality 590A1 Heavy Walled (HW) Barrel Heat Shields for some time now. They originally offered different length Heat Shields for both 18.5" and 20" 590A1 HW barrel shotguns.
It appears now they only offer the 18.5" Heat Shield which will work on both the 18.5" and 20" barrels.
AIMPRO Tactical has had a long-term working relationship with Mossberg. In fact, at SHOT Show, AIMPRO sets up in Mossberg's booth.
I suspect that AIMPRO provides Mossberg with their 590A1 HW Barrel Heat Shields.
Ref: "Rebel Rouser's" starting thread advised the Mossberg Catalogue Product Number for the 590A1 HW Barrel Heat Shield as "16335P".
As you can see from the pics below, AIMPRO's PN number for their 590A1 Heavy Barrel, 20", Heat Shield is "PN16335P".
The Heat Shields for the 20" Heavy Barrels cannot be used on the 18.5" barrels, as the Barrel Lug is positioned in a different position. That's probably why AIMPRO/Mossberg decided that the 18.5"Heat Shield will work on both models, and only offer the one now.
Below are photos of some of my shotguns with AIMPRO 20" Barrel Heat Shields.
1st Photo shows a comparison of [TOP] Benelli M4, with AIMPRO 20" 590A1 Heat Shield (Fits fine - with a little modification to the hand guards.); [MIDDLE] Mossberg 590A1 with 20" AIMPRO 590A1 HW Heat Shield; and [BOTTOM] Mossberg 590, with Mossberg, factory 20" Heat Shield.
2nd photo is a comparison photo of the HW Barrel 590A1 18.5" Heat Shield against an installed 20" AIMPRO Heat Shield on 590A1 HW 20" Barrel.
3rd photo shows comparison of AIMPRO 18'5" and 20" Heat Shields.
Last photo shows close-up of AIMPRO packaging for the 590A1 18.5" and 20" HW Barrel Heat Shields.
Cheers!