This Accessory Mounting Block has five 1/4-20 tapped holes on it. You can screw any male 1/4-20 onto it and mount it just about anywhere on any 15mm tube.
These are great, especially along with the bored version. Great to have a bunch of these around for outfitting your rig just how you like it. Machined to perfection as is the norm with all Berkey's work.