The torquemeters in the DC-6 are calibrated in psi of Brake Mean Effective pressure (BMEP) which is intended to be the mean pressure acting upon each piston, minus mechanical losses, which is a particularly arcane and useless quantity to all but an engineer designing engines. But, like Lord Lucan said, it's just a number, representing a certain value of torque. know what number you need to see on the gauge and it doesn't matter what the units are.
Other Pratt and Whitney radials had torque meters calibrated in PSI of torquemeter pressure, and the actual measured quantity in the torquemeters in P&W radials was oil pressure in a set of hydraulic pistons in the gear reduction drive. Again, a completely meaningless unit to the pilot.