The ornamental design for a slide rule plaque, as shown and described.
FIG. 1 shows a front view of my design for a slide rule plaque according to the invention;
FIG. 2 shows a top view thereof;
FIG. 3 shows a bottom view thereof; and,
FIG. 4 shows a side view thereof with the opposite side view being a mirror image.
The name plate and school seals are show in dotted lines and form no part of the claimed invention. The back of the plaque forms no part of the claimed invention.