Apart from the insides of the reg there is only one seal under pressure. It is highlighted in Rob's guide - it seals both the reg to the action and acts as valve seal.
Does the leak also occur when you put lateral load onto the air cylinder/reg body? Either way i'd suggest (only if you feel comfy), removing cylinder, emptying reg of pressure and checking the 2 hex bolts that hold the reg on. If the leak persists i can only assume the reg/valve seal is at fault and requires replacement