I think the screws for linkages in the Duo-Art piano may be known as
'shoulder screws'.  These are very common in old players and in many
nickelodeon players such as the Wurlitzer roll changer.  These screws
are easily available or you can make your own with drill rod stock and
a lathe.  I prefer to buy them made.  I buy them from McMaster Carr
with a slotted head.

There are also shims available for these screws, available for number
10 and above.  One example is the 6-32 shoulder screw with a 1/8-inch
shoulder that is 5/32-inch in diameter which can be found as part
number 91829A523, for $1.56 each.  A common screw in the Wurlitzer
machines is a 10-32 1/4-inch shoulder with a 1/8-inch high shoulder
as part number 91829A531 for $2.03 each.

The shims are sold as 'shoulder screw shims'.  Shims for smaller sizes
can be made with a Roper Whitney hand punch and tiny fingers to hold
them.

Don Teach