Brass radiator plug for the D1 fits it. I have two of these on my diffs. Just use a little sealant of your choice to make a nice gasket to go where the O-ring was on the plastic plugs. I put the sealant around the threads at the bolt head, and then taper it from the shaft to the outside of the bolt head. $3.99 each.

http://www.rovahfarm.com/DiscoICooling.htm