Finding a piano from this era with original ivories that have been
replaced with plastic is extremely common.  They often become chipped,
worn from heavy playing, and discolored.  While moderately damaged
ivory keyboards can be repaired, a complete replacement set will cost
a small fortune.  At present the only ivory that is legal to have is
the so-called "pre-ban" ivory, material that was stockpiled before
it became illegal to import.  You can still get a complete new ivory
keyboard but the supply is rapidly dwindling.  Consequently the cost
has skyrocketed, now well over $5,000 last I checked.

Keys are _not_ interchangeable from piano to piano, even the same exact
model and year.  Each one fits slightly different and the key pins
never quite align the same.  Consequently if you want to move keyboards
from one piano to another the only practical way is to swap out the
entire key frame with it.

This leads to new problems, however, in that the key frames do not fit
exactly the same.  They will need work to make the action stack fit on
correctly and then you may find that the wippen heals do not align the
same with the key capstans.  Likewise the frame rarely (if ever) fits
the keybed the same and will require work to make it sit perfectly flat
so that it does not knock and can be adjusted correctly with the glide
bolts.  This also assumes that the key tails will align with the damper
action levers the same.

I have successfully swapped out a complete keyboard from a modern piano
that had detrimental problems with one obtained from the manufacture.
They were made largely with modern robotic CNC machines and yet still
did not fit the same and required an entire day of work to get it to
work correctly.  I should add that Mason & Hamlin pianos are extremely
popular for rebuilding.  Removing keys from one Mason & Hamlin to put
on another would be silly, as the demand for unrestored ones is very
high.

The keys that you have were cut down intentionally.  This is because
modern plastic key tops are thicker then ivory and therefor need to
be trimmed to accommodate the added thickness.  The key weights were
probably just installed unusually high at the factory.  It is also
possible that whoever installed the plastic tops was not the greatest
craftsman and did a crude job.  Nevertheless the keys you have are
probably salvageable.

Complete key rebuilding is a common practice, even ones that were
previously worked on badly.  Wood can be built up using veneer and
then resurfaced, bad key mortises can be routed out, plugged, and
recut, and bad key buttons can be replaced.  As mentioned before,
even new ivory is available if you don't mind the expense.  I am now
in fact salvaging ivory from old junk pianos to reuse on my own future
projects.  I cringe at the thought of how many ivory keys I have thrown
out over the years but in the past it really wasn't a big deal.  Now
salvaging them for reuse has become worth the trouble.

I can recommend someone who specializes in rebuilding keys if you are
interested.  More then likely you will want to stick with plastic tops
however.  The economic justification for returning them to ivory is not
likely there.  If the plastic tops have been put on poorly and the
weights are bad that can be redone.  Keep in mind however that key
weight adjustment can only be performed after the action is rebuilt
with new parts.  It is a process that requires proper balancing and
some experience.

I would start by having a local very qualified piano technician inspect
the keys for you to get an evaluation on what should be done.  If it
is determined that they can be rebuilt properly send them to a key
rebuilder.  Then have the technician return to balance them properly
for you.

Rob Goodale, RPT
Las Vegas, Nevada