"Micro Seiki CU-500 is super heavy (2.7kg) pure copper mat, very rare and very expensive (over $1200), better than SAEC SS-300, but not for every turntable. I use it on SP-10mkII and they are looks like they are made for each other. I remember when i replaced stock rubber mat, the effect was huge! It was the biggest upgrade for my SP-10mkII. CU-500 and thinner, lighter and cheaper CU-180 does not fits the Luxman PD-444 well, so it is always important to measure the platter and mat before buying any of them.
I'm fine to put the records on the metal mat, i think the engineers of Micro Seiki knows well what they are doing. The platters of Micro turntables are copper. I don't have the ST-20 clamp, it's extremely rare, but i use ST-10 instead. I think the ST-20 was made for CU-500 mat."
Slight correction, the Micro Seiki CU-500 and CU-180 are (were) made from gunmetal copper alloy.
Only a handful have ever made one from pure Copper, like the Artisan Fidelity Pure Copper Universal Platter Mat (3.6kgs.). Robusto used to make one, but had to switch to gunmetal copper alloy they said to reduce production costs.