Aluminum pony bottle bracket. Multiple uses for this style of bracket but primarily they are used for mounting a pony bottle (generally 4-5" diameter bottle) to your primary cylinder. Hose clamps go around the pony bottle and you can either use the cam bands from your BCD to strap to the primary tank or some people will opt to add another set of hose clamps and semi/permanently mount the bottle to their primary cylinder or doubles. Get creative, don't die.
