If you are looking for early SOPMOD / SOCOM clone-correct builds, like the Mk18, the Gen1 LMT stock is what you need. No longer produced, this is the stock that was used by the US Military.
Note: Gen1 stock is no longer available from LMT. We get very few in each year. Also, the exact same stock is available from B5 here: B5 SOPMOD Gen1 stock
Gen1 stock does not have the QD sling connector, so slings were attached through the slats in the back and tied on. Later, LMT introduced a Gen2 stock which had QD connectors.
Hard to find, clone correct.