We used to sell them but no longer do. It lost it's popularity due to higher demand for mid-length gas systems, low profile gas blocks, and long rails to cover them. We don't even offer the DDM4 V1 any longer for the same reason.
We do, however, still offer the RIS II M4A1 FSP and the newer RIS III 13.0 FSP. One M-1913 and one M-LOK option. Both of there are far easier to install that the DDM4 Rail 12.0 FSP due to the removeable lower rail.