John,  I've seen several installations of the "steamboat" pump with
slightly different electric motors, and the motors have all been
suspended in roughly the same way.  I've attached a low-resolution
picture of one in a 1917 Steinway I restored earlier this year showing
a little bit of it.  Unfortunately I didn't take any really good
pictures of the configuration, and the piano is now 2000 miles away
with its owner.  But hopefully this will help you get yours properly
suspended.

Yes, the rear of the motor mounting board is suspended on a wide
leather strap or "cradle" with two holes near each end, slipped over
the large vertical metal pins in the cradle frame, as you describe.
The right front corner of the motor was originally attached firmly to
a wide leather strap (about 1.25" wide by 0.25" thick), with metal
grommet holes near the top that hook on the adjustable metal hook.

The hook is adjusted in its holder by a wing nut, padded with
a felt bushing (green in the picture), and the effective length of
the supporting strap is adjusted (coarsely by choosing the closest
grommet hole, and finely by adjusting the wing nut) until the motor is
approximately horizontal when the pulley on the left side is hanging
in its belt to drive the pump.

The main function of the leather strap is to support the weight of
the motor that isn't supported by the drive belt.  A second, similar
but slightly differently bent adjustable metal hook with wing nut is
installed at the left front corner of the motor mounting board, to
adjust the tension of the belt.

Typically, the hook is fastened through a sturdy metal staple or heavy
eye-hook fastened into the bottom board of the piano, and the wing nut
(also padded with a felt bushing, green in the picture) is placed so
that it pulls down on a pierced metal strut screwed to the mounting
board.  When properly tensioned, the left-side hook just keeps the belt
snug so that there is no slippage, and it keeps the motor from bouncing
upward while running.

Also, in many installations I've seen there is a large rubber-head nail
driven either into the bottom board of the piano centered under the
motor, or in the bottom of the mounting board, so that the motor has
something soft to rest on when the motor is unhooked from both the belt
and the front support strap.  It helps during maintenance, since the
motor isn't suspended very well on either the front strap or the belt
alone, but the rubber button should generally not come into play during
normal running.

As the picture also shows, I generally find that a single modern
industrial V-belt is adequate between the motor and the pump rather
than the two original (usually sewn leather belts), and two (rather
than four original) are adequate to drive the slow cam shaft.  With
fewer belts, the pump runs with less friction at slightly higher RPM
and output, and with much lower noise and motor strain.  And it also
reduces the sideways load on the shaft bearings.

Best regards, and I hope that helps.

Ralph Nielsen
http://www.historicpianos.com/ 

 [ Electric Motor Suspension for Steamboat Pump
 [ http://www.mmdigest.com/Attachments/13/11/20/131120_223233_Steamboat_Pump_Motor.jpg