TAHK, вот я нашел описание этой пимпочки.
Фотки есть на http://strannik.io.com.ua/album.php?ida=814, сорри но у меня что-то ББкод не работает - не получается вставить картинки.

P90 SLING HOOK UP POINTS:
Because our P90 comes with one both sling hook up points the only reason you would need these is if you bought a P90 from somewhere else. These are available only through MFI as we manufacture them.
These are the same sling hook up points that MFI makes for the REAL FN P90!

This is how these work:
The Front Sling Hook Up Point is a flat piece of contoured (1/8th inch thick) steel with a circle cut for the barrel and a slot cut (1.18 inches wide) for the sling webbing soft point to go through. You remove the flash suppressor and slide the Front Sling Point over the barrel and up to the end of the barrel. The circle diameter is large enough to allow the sling point to move freely around in a 360 degree circle making this sling point 100% ambidextrous. Thus it does not matter whether you are right or left-handed!

The Rear Sling Hook Up Point is a loop of webbing designed to be slide through the lower receiver sling slot of the P90 and then around the top again where we have attached a loop (ballistic nylon) for a sling soft point to go through. Again you can move the lock / tightening tab to the right or left side without changing the position of the loop at the top. This makes the real hook up point also 100% ambidextrous! The reason we located the rear hook up point at the top is so that when the weapon is in the patrol position / at rest it stays perfectly horizontal which allows the user to fire from the hip with out adjusting the elevation of the barrel. Ideal for check points where you want a weapon's system pointing at all times into a vehicle while you check identification. If you so choose the top loop can be moved / adjusted anywhere around the butt in 360 degree!

SPECIAL NOTE: Given the short length of the P90 and the position of our sling hook up points you do NOT need a expensive tactical sling for the P90 as you can go from rest / patrol position to shoulder firing position using a standard (non-tactical) sling!