These viewers are "new" and still are being sold. The lenses are single-element plastic lenses but are suprisingly good for a viewer that costs so little (under 5 U.S. dollars each). I personally have bought at least a dozen to "have around" and to give away (along with slides) as gifts. The stereo slide seen in the viewer photos above is in an RBT mount. Click the image above for the place where I bought these.