Renault Gearbox Red Plug

Help please

My Renault 21 turbo box bought new by the last owner and still never used has a red plug at the rear. This is plastic and I am certain would not last ,

What should be in there or do I just replace with a metal plug?

thanks
Ian

Ian I believe Renault used this as a anti theft devise that locked the gear box in reverse gear,it just needs a metal threeded blanking plug with washer to seal it off

Brian is correct.