Simple reveal, right? Guess again - that bevelled edge is actually a spacer strip of thin lath cut from the edge of a 2x6 to a 1/4" thickness! The lath was glued and tacked in place along the edges of the rough opening, then hand planed to bring the different edges all flush. The new stop pieces were primed and painted again, then I cut and installed the shellacked clear pine 1x4 trim boards with a quarter-inch reveal. I'll sand and re-shellac the mitered edge to finish it off.