These two Segments define the west boundary of the Park along Marble Canyon; O is shared with Kaibab National Forest, P with BLM lands in House Rock Valley. Here is the official map:
The map instruction puts the boundary in both Segments on the Canyon rim.
In 1969, when Marble Canyon National Monument was proclaimed by President Johnson, the boundary along O in Kaibab NF was set on the rim, but along P, a more complicated line was specifically drawn. It was set 500' back from the rim, and at the crossing of each side canyon (North, Rider, Badger, e.g.). the definition was on legal lines. This finickiness was dropped along with the 500' setback in the legislative versions. But in doing so, the boundary cut too far in across certain prominent side canyons. The remedy was to carry the boundary back along the side rim so as to include the side canyons. Here is the version that John McComb worked up for presentation to Congressman Udall, who was trying to remedy the shortcomings of the Goldwater version:
The numbers are acres added. These changes were kept when the bill passed the House and was reconciled in conference and the Act passed. (The map also shows the addition of the river stretch from Navajo Bridge to the Paria, left out of the Senate bill.) This map shows the boundary correctly along the left or east bank of the river along the Navajo Reservation.
The BLM, AAA, and NG/USGS maps all have the wrong boundary on the east side of Marble. The Kaibab NF map is correct, but does not go all the way north; so here it is:
And just for fun, here is a corrected (though somewhat sloppily) BLM map for the Canyon north of Bedrock:
And that completes our tour of the Grand Canyon National Park boundary segments.