As I and others suspected, it appears to house a GPS antenna for aligning the weapon's GPS/INS prior to weapon release.
https://x.com/John_A_Ridge/status/1802786533864284401
Something like this could be adapted to HIMARS launch vehicles that are used to launch GLSDBs.
https://x.com/Azovsouth/status/1802831326892159284
Awaiting further information...
I wonder where the S-500 is located.