It's a verifiable double check valve. The removable test plug is there to check that the valve is working. If you depressurise the upstream section of pipework and open the test point, water should not flow from the opening which shows the valve is closing and preventing backflow from the downstream pipework.
I have the same one on my desk here now. It can be replaced with any kind of modern double check. The only issue is finding one the same length otherwise you may need to alter the pipework to get anew one to fit.