The P51 is the power amp which accompanies the C51 pre.
Weighing about 10kg, the unit is able to deliver 50W RMS per channel and had the innovative feature of being able to be converted into a mono-block with the flick of a switch (for the 60's).
The P51 is known to be able to drive extremely demanding loads and many often used them with ESL(s).
In addition, the C51-P51 pair was often the reference set for many reviewers in the late 60's and early 70's.
Need to locate the special DIN cable which came with it ... one of the joys of having too many units and managing the misc cablings for them!!!