The hammer shank end caps are the small plastic washers that are screwed on to the ends of the hammer shanks to hold the hammer heads onto the shafts.
You need one hammer shank end cap per note. All of the hammer shank end caps are the same.
Print the hammer shank end caps upside down, with the wide end at the bottom and the narrow end at the top.
Build plate adhesion is strongly recommended. These are small parts that would easily come unstuck from the build plate otherwise.
Since these are small parts, consider printing many parts at once to improve throughput by reducing the amount of time wasted waiting for the print bed to warm up and cool down. If you really only want to print one or two of these parts, then you may need to adjust your printer settings to increase the time between layers to give time for each layer to solidify before the next layer is added on top.
The default 20% infill works well.