Eddie's Helmet (Research)

Per a discussion in the Rocky Horror Costume FB Group, it appears to be a M16 Stahlhelm (issued from 1916 to the end of WW2) likely with Lancers Death or Glory Badge on the front. Replicas available from Epic Militaria in Ireland (however it doesn't come in silver color). 

Ruth's research on the Anal Retentive Costume List claims it's a German WWII M42 style helmet, which is similar with slightly different holes.

screen shot provide by Peter J.