It's also worth noting that once U.S. Regulations change, contractors and suppliers are then left with stocks of now-unwanted merchandise. That's why U.S. Government contracts stipulate numbers of items to be delivered and a time frame in which they are to be delivered, so as to not "burn" manufacturers whose services they may need in the future. That being said, for various reasons stocks of now-dated items would exist in the hands of either the government, manufacturers, or suppliers; the answer: dump them at attractive prices on the National Guard, state militia, or even Masonic orders who weren't subject to those same regulations!
The shako in question *might* be an original U.S. Marine or Mounted Artillery model that remained in unissued stock and been disposed of, only to have the non-regulation starburst, GG insignia, and side buttons attached at some now-unknown but later date. A friend and mentor of mine was an absolute genius at recognizing things like that at our favorite HUGE outdoor flea market - once I asked him WHY he'd bought a dozen ugly dark blue caps with Woodmen of the World insignia on them: he then pulled back the sweatband in one, revealing the U. S. Ordnance Dept. ink stamp. The entire lot were discontinued M.1902 U. S. Army dress caps, which he subsequently refit and sold at handsome profits!