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The PG7307 piston gauge is a special model piston gauge -- a controlled clearance, hydraulic piston-gauge base.  The piston-cylinder and base are not interchangeable with the PG7302 piston gauge.

http://us.flukecal.com/literature/product-manuals/pg7000%E2%84%A2-piston-gauges-operation-and-maintenance-manual