This directory exists because the SimReady prop sublayers under external/omniverse_mirror/simready_props/<name>/ bind their MDL material through the relative asset path ../../../materials/SimPBR.mdl (e.g. antiquelvasesmall_inst_base.usd), which from that location resolves to external/materials/; without it, renders log the MDL error "could not find module '::..::..::..::materials::SimPBR'" and the affected props (50 of 280 pool assets) render untextured. The contents are byte-identical copies (sha256-verified) of the 28 mirrored files, plus the mirror_manifest.json documenting them, from external/omniverse_mirror/Assets/simready_content/materials/, whose manifest records the upstream source https://omniverse-content-production.s3.us-west-2.amazonaws.com under prefix Assets/simready_content/materials/ (mirrored 2026-07-07). Copied on 2026-08-18.