They are UNS mounts for night vision optics.
PGWDTI.
As an aside the forward mount on the left is the Canadian Forces PEQ rail which only has rails at 3 and 9. Why the CF elected to not have a rail at 12 is a mystery...
The one on the right is the mount we designed and produced for the USMC M40 fleet. Its official designation is "M40 Accessory Rail". It has rails at 12 and either 3 or 9 (reversible). Every USMC M40 either has this or is getting it.