Well, let me share what I've learned while you're searching...
I found two other manufacturers that are currently making +2 mag extensions for the Mossberg 590's. The +2 is the perfect length to extend the stock magazine on 18-1/2" barreled 590's to the end of the muzzle, making them 7+1 capacity.
Choate Machine and Tool makes a steel one in matte blued finish (upon request) for $45
Nordic Components makes an aluminum one in matte black anodized finish for $80
I was trying to reach Gun Connection because it looks like his custom extension is parkerized to exactly match the 590A1 shotguns.
Hope this helps someone else : )
Thanks,