Casque Facts:
First found: Chicago, Grant Park 1983
Second Found: Cleveland, Greek Gardens 2004 
Third Found: Boston, Puopolo Park 2019

The contract to build the casque was finalized on June 26, 1981. After that date the first casque would be sculpted, then sent off to be molded for a production run. Of that production run all 12 casques would need to be produced, delivered, and painted, before they could be buried.

Initial Contract

Only 12 casques were contracted

All Casques have been identical. 
All Casques have been signed.
The Casques seem to have not been fired.
The Plexiglass is a very specific type.
The Cleveland Casque had metal in the lid and key.
The Chicago Casque has metal in the lid.
The Boston Casque had no metal, and has burlap in the lid.
Each lid has a different time.
The time matches the Puzzle month
Each lid has a unique painting.
Each Casque has a unique but specific figure painted.
Plexiglass box size: about 4.7/8, 5.5/8, 6.7/8
Each Plexiglass box is stamped in a specific way.
Chicago Casque depth: Less than 12 inches to the top
Cleveland Casque depth: About 18-20 inches to the top
Boston Casque depth: Unknown