PX7-377
From The Stargate Omnipedia
Likely a reference to PX7-377, the planet where SG-1 encountered a crystal skull with phase-shifting technology. Cameron Mitchell seems to have misspoken when he referred to this planet as "P7X-377."
KEY EPISODE
Arthur's Mantle - Mitchell references the crystal skull planet as "PX7-377" instead of "P7X-377."