I believe the muzzle brake comes with a 'choke' to make the brake/breacher screw in and the other is Cylinder Bore. If you didn't buy other chokes, there aren't any besides Cylinder Bore, from what I understand.
http://www.theshootingedge.com/products/fab-complete-door-breacher-kit
Product Details
Fabarm Complete Door Breacher kit for STF 12
Product Features
Kit Contains,
Breaching Break, w black finish.
Extended Inner HP Accuracy Choke to attach the Brake.
I got an email from the TSE's gunsmith (who Ive been told is the man to ask). Like you said, he stated the "inner HP" choke that comes with the shotgun and supports the muzzle brake is a Cylinder, safe to fire slugs through. But oddly, stated the other choke which has no muzzle brake thread, that also comes with the shotgun is more like a Modified, which you cant shoot slugs through. Whats odd is that the provided parts diagram that comes with the shotgun labels this choke as E-340 (HP CYL.)... which id normally assume means the choke is a Cylinder as well ??