Looks to be the Mk 134 Mod 0 motor which is used on the ESSM and AMRAAM-ER and is capable of Mach 4+ when ground launched ( my bug report was accepted for AMRAAM-ER).
It looks remarkably similar to it, but this could be a new version of it considering how new this is vs when the RIM-162 entered service
could be a motor designed off of it too and has completely different internals
From some other sources, it almost seems like a pruned down AIM-174 / SM-6 to fit into a F-35’s payload bay with a dash of ESSM.
While I don’t think it will sport the max range of the 174 and SM-6, this seems more in line with a a larger cousin to the AIM-260 with maybe the 174 still being reserved for sniping AWACSes and such.
I have a feeling this will be the replacement for the AIM-174B, and the 174B will merely just be the stopgap until this enters service. The 174B and 424 are both 13.5 inches in diameter, and they have similar weights, while this has even more range.
Might be a bit out of left field but I feel this might be our first HGV AAM to enter service, considering every other HGV has been a hypersonic ballistic missile like Mako or Blackbeard.