Marshall gas regulator
 

Does the black plastic leaver that points to the tanks do anything? Mine works even when not pointed to the open tank. Also the red/green sight indicator is it help to red fill level on tanks? thanks

It might be the automatic change over that will switch tanks as one is used up.

The black lever indicates (points to) which cylinder is in operation. For example, if the lever is pointing to the left tank and the indicator is green on top, that tank is in service. If the lever is pointing to the left tank but the indicator is red, the regulator has automatically switched over to the spare (right) tank and it's time to refill the left one, as it is empty.

So it does not shut off gas flow to the tank it is pointed at? Ok I thought the regulator may be broken because there is gas flow in both directions. thanks

My understanding is that, once it switches over to the new tank, it cuts off the old tank. The old tank's valve should be turned off manually at that point. The black lever will need to be manually switched over to the new tank just so you know which tank you are feeding from. When you do that, the indicator should turn green.