Triad Aluminum Slip-In Stereo Slide Masks - Box of 100 - 5 Perf. Close-Up - NEWTriad Aluminum Slip In Stereo Slide Masks Box of 100 5 Perf. Close Up NEW Manufactured by one of the two most successful stereo projector manufacturers, both located in Chicago, Illinois, the Compco Corp. It seemed perfectly natural that Comco would gladly develop a unique concept in stereo slide mounts the patented three piece, laminated slip in stereo slide mount. A slightly sturdier aluminum mask than the competition, the Triad masks feature slots