"Peculiar".. to me the word describes something out-of-the-ordinary, synonymous with "Esoteric" or "One-of-a-kind". Seeing or hearing of the word Peculiar also reminds me that it is contained in one of the more ambiguous acronyms to ever come from an even more esoteric entity in the US Department of Defense- the United States Special Operations Command (USSOCOM). This command was responsible for sending out a directive to upgrade and modify the M4 Carbine- which originated from the venerable M-16 "black" rifle of the 1960's. The upgrade was known as SOPMOD.
SOPMOD, or Special Operations Peculiar MODification was borne of the apparent need to create an upgraded version of the base M4 Carbine with additional low-cost accessories added-on via a modular quad-rail picatinny hand guard. This initial offering would be known as the Block I version- later versions and additions in accessories would earn the Block II designation.
Building up the M4 Carbine used by non-standard elements of the US armed forces would include added optics, sights, target designators, (visible and infra-red laser pointers) weapon lights, (visible and infra-red light beams), the SOPMOD Stock, and even foregrip and grenade launcher.
Yes, even the very simple and utilitarian collapsible buttstock received a place in the SOPMOD modifications to the M4 Carbine. This specific piece of hardware I believe received it's share of modifications that make it one of the more "Peculiar" additions to the array of accessories of both Block I and II versions. The SOPMOD stock at first glance is noticeably wider in size compared to its slim-line cousin the collapsible SP1 buttstock found on the original CAR15 short barrel carbine; both sides of the SOPMOD stock was built up in surface area in order to provide not only a more solid cheek-weld for the shooter- which creates stability in keeping the eye focused on the sights when engaging targets- but also added battery storage compartments in order to accomodate the additional electronic optics accessories of the SOPMOD M4- incidentally, the M4 military designator also was given a new moniker of M4A1, distinguishing it as a modernized and beefed-up version of the M4 Carbine with standard and shortened barrel lengths.
Having a weapon such as the M4A1 required the modified SOPMOD stock to address the need for better control of a carbine, with slightly heavier overall weight due to the added accessories. For example; the butt pad piece of the SOPMOD stock is considerably thicker in dimension and composite to better absorb the recoil of both the hotter load of the 5.56 NATO, AND that of the M203 grenade launcher. Recoil factor and the added weight should have been a fatigue concern with the shooter; fortunately, with the slightly added mass of the SOPMOD stock, the resulting front-to-rear weight distribution (with the slightly heavier SOPMOD Stock) created a better COG or Center-of-Gravity for the weapon overall.
In summary; the dramatic look, fit, and feel of the highly modular M4A1 with its high-tech accessories and additions, were actually rounded out and finished-off with the very low-tech and simplistic SOPMOD stock design, even down to the addition of the game-hunter's simple, tried and true, quick-detach sling swivels.