Q: Is the inset part of the mount (the cup where the post sits) made of stainless steel?
A: Nope, it's made of aluminum. Only Tactical Link sling mounts have a stainless steel interface. Thanks for the inquiry.
A: The Bravo Company BCM Gunfighter Lightweight KMR KeyMod QD Sling Mount With Rotation Limiter is an almost identical sling mount that is made of steel (instead of aluminum). Here is the link to the product: https://www.tacticallink.com/Bravo-Company-BCM-Gunfighter-KMR-Keymod-Lightweight-QD-Sling-Mount-With-Rotation-Limiter.html