There should be a brass screw with a brass sleeve over it on the
lower right, front of the transmission case.  The sleeve should
turn to reveal an opening.  The oil level should be at this level,
no higher or it will run out of the shaft ends.  SAE 80W/90 gear
oil is appropriate for the transmission.

Another way to check the lubrication is to use a mirror to inspect
the bevel gear through the rectangular hole in the top of the case.
When running the gear should have a fluid covering of oil.

A rather delicate ball bearing in the transmission can easily be
destroyed if allowed to run dry or with dirty oil.  It may pay to
clean it out.  Just be sure to keep all of the parts straight if it
is disassembled, the shims and bearings may look the same but they
are not interchangeable.

Be careful when moving the piano also, if it is tipped too much
the oil will run out and make a mess.

Larry Emmons