JB, Technics offered three plinths over time for the SP-10 Mk 2 and 3 series
motor units. Yours, the SH-10B3 was the lightest at 26.5 lbs. Next, the SH-
10B5 weighed about 38 lbs. and the last one offered, the SH-10B7 was 42
lbs. I can't be certain of this but it makes sense that the Technics engineers
discovered that more mass helped performance, thus increased the weight of
their plinths over time. But I should mention that more mass was needed for
the Mk 3 since it has more torque.
I have read elsewhere that around 45 lbs. should be considered the minimum
weight when building a plinth for a Mk 2 or 3. Look at the weight of your
plinth and draw your own conclusions.