General Information: The application of US Marine Corps cap badges to steel helmets was a common practice in the post-World War One period, but was this was also done, to a lesser extent, during the conflict. The debate among collectors about whether EGA (Eagle, Globe, and Anchor) badges were used during the First World War was been settled by the study of period photos (e.g. see photo above). It seems that practice was more common among USMC officers, but was also done at least occasionally by enlisted men.
Displayed Example: I bought this helmet in 2015 at the Show of Shows in Louisville, Kentucky. It came from an old collection that the table holder was liquidating on behalf of a friend. The helmet is a British Mark I, the type issued to early-arriving members of the American Expeditionary Force (AEF). The EGA is the version used during the First World War. The hole for the badge was punched rather than drilled. At that time, soldiers and marines generally did not have access to power drills, and it would have been difficult to perforate the helmet with a hand drill; therefore, the holes were punched with a hammer and nail, or something similar. It is impossible to say whether the helmet was in its current configuration prior to the Armistice, but it has the right characteristics of the type (WWI vintage EGA/punched hole). The fact that it is an earlier-issued British MKI type helmet makes it more likely that the EGA was on the helmet during the war since virtually all the MKIs used by Americans were issued to early-arriving AEF troops. US Model 1917s would have been somewhat less likely to have an EGA attached during the war because many of these helmets were issued after the Armistice, or remained unissued.
Collector Notes: Helmets with collector-installed EGAs abound and this can be a tricky thing to detect. Look for First World War type EGAs and punched versus drilled holes. Helmets with drilled holes may be original, but they are likely to be later, post-war vintage. Of course, there are perfectly original M17s and M17A1s with later vintage EGAs that were used by US Marines during the 1920s and 1930s, and these are interesting historical objects as well.
* “Period Photo’s WWI USMC Helmets w/EGAs” Post #46, “USMC officers that had survived the battle for Belleau Wood. The photo was allegedly taken shortly after the capture of Belleau Wood.” Devildog34.. US Militaria Forum. July 5, 2015. Accessed April 14, 2022. Original citation sought.
 ”Period Photo’s WWI USMC Helmets w/EGAs” from US Militaria Forum. Discussion tread started March 17, 2015 by “Maple Creek.”