Overhead Projection Student Slide Rule
Frederick Post model 1447DP [1969-71]
Teledyne-Post model 44AA-47R [1972+]
Hughes-Owens model 341 3734

Probably made in USA to Post's design.

Post's three overhead projection slide rules first appeared in the 1969 School Drafting Supplies Catalog, did not appear in the 1971 catalog and reappeared in the 1972 (Teledyne Post) catalog.

Each slide rule consists of two 12 x 7-inch sheets of lucite.  The bottom, stationary, sheet is printed with the scales from the slide rule stators.  The top sheet, which can move left and right, has the scales from the slide.   A 2-1/2-inch wide piece of lucite with a vertical hairline slides on top.  Light from the overhead projector passes up through all three layers.  Each rule came with a set of vinyl sheets which, when placed over the rule, mask out all but two or three scales.  These slide rules came with a one-page instruction sheet in an unusually sturdy cardboard box lined with red flocking.  The flocking leaves red dust everywhere.
!969 School Drafting Supplies Catalog, p 25

The rule is supplied with five masks.