File:BSI-Shell DIFF.png

Summary
Hidden on the bottom of gun shells is:

"P.O. Box 690412 Quincy MA 02259 Irrational Games PP Watson OC-87 P.O. Box 690412 Quincy MA 02259"

When searching this P.O. box, it is found to belong to this location: http://www.quincyquarrymuseum.org

This is likely a location where Irrational Games obtained some of their period imagery.