Hi Gordon: I'm not all that familiar with the Standard pedals, but
if there is a metal frame surrounding the rubber mat then it should
be removed first.  After all these years, I would be reluctant to try
to separate the mats from the blocks, although soaking them in alcohol
would soften and dissolve shellac -- but that is assuming they used
shellac.  Maybe they used hot glue, in which case warm water will
remove the mats.

If you keep the mats on the blocks, then you can do the following:
Make the mold of the mat and block.  Then, when it is time to make
the new mats, take the mold and carefully pour in the casting material.
Don't fill up the mold; just pour in more than 1/8" and less than 1/4"
of black rubber.

Obviously, the mold must be on a level table to be able to pour a new
mat with uniform thickness.  It would help to have a really accurate
scale because then you can make up 20 to 30 grams, and experiment to
see how many grams of liquid make a mat that isn't too thick or thin.

Randolph Herr