@poorguy I never learnt what the metal was that is used on the AT-666.
I know others use this mat on this forum, and it might be possible to learn which metal is used if a wider inquiry is made.
I also know through this forum, it has been used without a Vacuum and has been rated as an attractive method, I have not heard the mat used in such a way, but would try it out in the near future, as I will be visiting the owner of the one I have tried out.
Again there are AT 666 owners on here who can offer a description with almost immediacy if willing to help.
The AT 666 is not always going to be able to successfully hold a Vacuum for a full period of a side of a LP, their age is contributory to this, the length of time the Vacuum is in place is a unknown, some do not produce a Vacuum anymore.
With the above in mind, it does suggest careful selection is required, when purchasing, if a Vacuum is the important function for the mat.