These parts are supplement pieces for the Boom Lift - 1-87 HO Scale.
These parts are for the boom and are optional. A builder can source these parts and use styrene instead if they wish which can be obtained at most hobby stores.
This was done to keep costs lower but as an option a modeler can now order this.
Builders will need to sand these items as they are designed for a tight fit. Paint and such will need to be taken into account for friction and a sealer should be used to prevent the paint from being rubbed off. Be sure to follow Shapeways instructions and do not use sealers or paints that can damage the printed item.