I found this picture that claims to be of US Marines at Guadalcanal that shows them wearing a mixture of M1917 Brodie Helmets and M1 Helmets, which prompted me to ask this question.
Until the transitition to the M1 helmet beginning in 1942, all overseas troops would have been issued this helmet. I've studied this picture, and subsequent others from the Guadalcanal campaign. This image I believe depicts Marines from the 8th Marines, 2nd MarDiv, coming out of American Samoa en route to Guadalcanal. There are multiple images depicting the M1917 alongside M1s there but seem to all be from the 2nd MarDiv. It's certainly possible units had a greater mix during the transition, especially for Marines. They notoriously got the newest equipment later than most, hence why the guys on the cargo net carry M1903s.
