That's some pretty deep pitting ! The Evaporust will work but you will be left with large pot marks in the steel. I have you Evaporust in the past and it is excellent stuff ! And it claims it does not harm brass.
If you got a M15 Jager Zu Pferde with the wrong eagle, here's a correct one for auction....
https://www.ebay.fr/itm/226649711290?mkevt=1&mkcid=1&mkrid=709-53476-19255-0&campid=5338722076&customid=&toolid=10050
This helmet is ALL wrong...
Just look at the stamping inside the shell....it say 1916. Therefore it should be a M15 helmet. It should have NO brass on it. Its a pieced together jager zu pferde helmet.