IMI range tribune dn22 temp and pressure reliefvalve

[Gary80gas]

Just looked at an old unvented cylinder with an IMI range dn22 temp and pressure relief valve. The valve has a side tapping about 1/4'' in size. Its plugged with a short brass fitting with a spade shaped rectangle end. Its leaking through the center which means it has another thread inside. Can someone tell me what its for? Many thanks in advance. Cheers

 

[chris watkins]

Anti-vacuum or at the port for one.