It looks like an S-series HBR-M, which is exactly what I used on these two replicas:
fyi, the free floating rail handguards I used in both of those models are almost the exact same length as an SR-25 / SR-M110 SASS forend.
The first picture of the SPR mk12 mod 1 used a replica of the A.R.M.S. #32 throw-lever bipod mount.
The second picture of the SR-15 Match used a replica of the KAC bipod mount.