It's too far back for a "Magic Box". It actually looks like it's accessing the air path out of the reg, but just before the on/off. So, a "Volumizer" type of thing, that stores extra post-regulated volume?
Drilled 'Cocker body? Cover those holes with the Eye Patch
! Doc's Machine
. Fixing, fiddling and transmogrifying markers since 1998. The Whiteboard
. Vaguely paintball-related absurdity every Monday, Wednesday and Friday.