A patent showing a metal & plastic resin mount for DSLRs is available to view. The plastic part of the mount seems to be for a much smoother mounting and dismounting of lenses. There could also be some weight and cost savings for lower priced cameras like the EOS Rebel line.
Patent Publication No. 2014-240858 (Google Translated)
- Published 2014.12.25
- Filing date 2013.6.11
- Of bayonet mechanism mount
- Mounting an inner peripheral portion of the body has a concave rail surfaces of metal
- Mount the outer periphery of having a rail surface of the convex shape in the resin
- That resin rail than the metal rail, I have a narrow width and high height