Stratolaunch's Boeing 747-400, registered N949SL and nicknamed 'Spirit of Mojave', sits at Mojave Airport near the more famous Model 251's hangar.
N949SL was originally delivered to Virgin Atlantic as G-VWOW in late 2001 with the nickname 'Cosmic Girl'. In late 2015, G-VWOW was moved to Virgin's Galactic program and re-registered as N744VG. 'LauncherOne' made many test flights and aerial launches before Virgin Galactic shut down. Surprisingly, there was a customer for the unique aircraft with its launching capabilities: Stratolaunch! It was moved to Stratolaunch in 2023 and was eventually re-registered as N949SL. It does the same work that it did with Virgin Galactic.
Also seen
HERE at Virtual Globetrotting.