Well thanks for asking Learner.
It's an old cylinder lapping block that works just perfect for stones. Weighs close to 40 lbs so it is solid.
Was for lapping cylinders true before mounting them back on rotary engine aircraft and the like. For finer stones, above 1k grit, I only use water on it. The natural slurrying of the stone quickly builds up in the grid to aid in levelling. For the 1k or lower stones I use a little 240k lapping compound to get the abrading started.
:tu