We currently know (or strongly suspect) that test bricks were made and used at the following locations.
Links to albums containing test bricks made by:
- LEGO
- System Plast
- Samsonite
- Courtauld’s
- Bayer
- BASF
- Borg-Warner / Marbon
- DSM
- Test bricks of unknown origin
[Original post and comments on Flickr.]