I received my new Mag extension for my Mossberg 590A1 18.5"
This is a new offering from CHOATE Machine and Tool. It is NOT (as of this posting) yet listed on their website. Choate calls it a "+1 mag extension"
It comes with extension tube, new longer spring, new Choate follower, and a barrel/mag clamp on sling mount. If you use the Choate supplied follower, it is indeed a "+1", But if you use the Mossberg factory follower - It is a "+2" tight.
I did, however, buy a Brownells Aluminum Follower(anodized orange), and a Wolff tubular magazine spring (40" long) and cut it to length -29" and bent in both ends.
It still is a plus +2 but with a bit more room, beacause Wolff spring dia. is smaller than the Choate supplied spring
So my Mossberg 590A1 18.5" now has a capacity of 7+1 :P