John,
here's the info on the Herbie's mat for the SL1200.
Way Excellent II-3.7mm:
For turntables with mat about 5/32" thick and for VTA-adjustable turntables with semi-hollow cast platters prone to ringing. Popular choice for Technics, Garrard, Kenwood, and Pioneer turntables. (For vintage turntables without VTA/tonearm height adjustment, use whichever Way Excellent II is closest in thickness to your present mat.) Overall thickness: 3.7mm; weight: 99.grams (3.5.oz.).
Size options:
11.56" (295mm) diameter, full-size mat.
11.25" (285mm) diameter. For platters with a raised outer ridge like Technics SL-1200 series and Garrard 401, undercut-bevel platters like Garrard "Zero," and other turntables requiring a thick, slightly undersize mat.
Sean