After seeing our photomask coasters, a friend gave us this giant photomask as an art piece. While we’re not certain what this was for, our best guess is that it is a mask for the lead frame for some device in an SO-20 package. The overall size is about 18″ x 24″ and about 1/8″ thick.
Shown with an SO-24 device on the edge of the plate for comparison. (Didn’t happen to have a correctly sized 20 pin device handy.)
The transparent lines show where the metal will be after processing and the black portions are where the metal will be removed. When you line up the 24 pin device on the plate it is a bit misleading, as it obscures the fact that those two outer transparent lines aren’t actually for device leads. It does confirm that it is sized correctly for a 1:1 mask for a device with a 0.050″ pin spacing, and the central square looks like the right width for a device in an SO-20 package as well.
Tangentially related: CCD wafer