From what I can find, the majority of the Brodie Helmets were produced in the UK. They made over a million for WWI.
The "ZD 24" is broken down as such:
Z is for American production, for WWI
D is the steel supplier (unknown on my end, sorry)
24 is the steel batch number.
As far as the insignia, I have no idea. I've searched a number of databases and found no matches. I would bet that it's 1: an American unit, based on production of the helmet; 2: it's probably long been retired. A trip to a well stocked library, or a letter with photos to The Institute of Heraldry (
The Institute of Heraldry | Home) might work; or 3: a long shot, but possibly a simple piece of artwork, meant to be patriotic. A number of Brodie's on the market have crazy art done to 'em.
Hope this gives you a starting point! Good luck!