My best guess is the cancel reads DUSTIN/OKLA. This is a cancel in a rectangular box, a type often found on parcels. It is not a precancel.
What measurements did you make? The design size may match, but the perf gauge measurement does not match a #596.

Comparing the left side with the bottom, there are two different gauges between horizontal and vertical sides. Do you know then what stamp this could possibly be?