its the mtm 3x magnifier. the black optex was made for a pvs14 but it has 3x demag and adjustments for collimation to correct for the magnifier. end result is net 1x like the utc so you can use a reticle or tracir
There are larger magnifiers that direct thread into the Skeet too. Whatever magnifier you go with on the front, you want an equal demag on the back end.
Image is slightly degraded when stacking the magnifier. Still very usable.