This one is the Polymer version.
"FEATURES
Advanced steel hinge design locks in both folded and extended positions
Pull-through design offering quick deployment
light-weight polymer (FS1913)
US veteran designed and proudly made in the USA"

Jonathon, FS1913 is the polymer strut model and FS1913A is the aluminum strut model. The aluminum brace weighs 13 ounces and the polymer model is three ounces lighter. The aluminum version is 9.25 inches long and the polymer brace is 9.6 inches. Both feature an identical, rubberized arm brace and stretchy, hook and loop-covered strap for attachment to your forearm. Besides the slight weight and length differences, the two braces felt nearly identical when used.

Jonathan, Polymer but, I would highly recommend the Aluminum one over Polymer here : finfeatherfur.com/sb-tactical-fs1913-pistol-stabilizing-brace-with-aluminum-strut-side-folding-sig-mpx-mcx-1913-rail-black/

I kept looking around and the aluminum version is the FS1913A out of stock at buds
budsgunshop.com/product_info.php/products_id/120990/sb+tactical+fs1913a01sb+sbt+side+folding+brace+with+1913+hinge+black />----------------
But in stock here
https://gun.deals/search/apachesolr_search/699618783148