SAEC's SS-300 mat is aluminum, anodised black and then coated with a very thin ( and very strong ) teflon coating
The success of the mat is in its cut-outs in the physical design as well as the coating which stops ringing and is gentle on records as well as being anti-static and balanced
Long out-of-production, I've tried dozens of mats and the SAEC is a keeper
I have the same feelings, it's one of the best mat in my opinion.
Here is the one on my Luxman PD-444, it's perfect for almost any turntable, i've been using it on Technics SP-10mkII and SP-20 too, it is also fine for SL1210mkII+ series, Victor TT-101 or Denon DP-80.
When i bought NOS sample in the box (with manual) i notices red tool and a small metal piece that allow to tighten up the mat to the spindle with a tiny screw (this is why there is a very small treated hole in the mat near the spindle hole). When you're buying a used sample these parts always missed and i believe a few people seen it.