You need the letter from the Princess' bedroom to look under Mordred's bed in the Museum. But the missing journal page says that the Pewter Moon is at "56 52", which in the saucer is put in as X-56 Y-52.
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The missing page is under his bed at the museum, and gives the coordinates for the Pewter Moon. The rest is just the island storyline.
x83 y49
Go to the nearby moon instead, at X-56 Y-52. (The coordinates are on the missing journal page.)
The coordinates are X 11, Y 82
Go to the Mordred Museum. Under the bed on the second floor is the missing journal page, with the coordinates to the Pewter Moon.
Mordred's journal is in the castle library, but it is missing a page, the one that gives the coordinates (X-56 Y-52) to the nearby moon.
Go to the throne room on the second floor of the castle.
The coordinates are X-56 Y-52 (from the missing page of Mordred's journal). You can only enter the coordinates after you have replaced the green fuel rod. (see related questions)
The coordinates for the Pewter Moon are on the missing journal page (X-56 Y-52). Once the fuel rod is replaced in the saucer Excalibur, you can enter these coordinates into the launch computer. (see related question)
From the missing page of Mordred's Journal, the coordinates are X-56 Y-52, which you enter on the dials of the Excalibur once you have replaced its broken green Fuel Rod. (see related questions)
The coordinates are on the missing page from Mordred's Journal (under his bed). The numbers are X-56 Y-52, and can be entered once a new fuel rod is in the Excalibur. (see related question)