The Duncan MOD Spacer kit 5158PK included a Dif-e-Yo concave ball bearing axle, two metal reversible spacers (patent pending), two plastic MOD Spacers that accepted Duncan SG (Silicone Groove) response pads, and a long, threaded screw/axle.
The reversible metal spacers let you use either the large (size C) concave bearing that was included with the kit to widen the string gap of your Duncan yo-yo and reduce its responsiveness; or, it let you reverse the metal spacer to let you use a small (half spec or size A?) bearing to narrow the string gap and improve responsiveness for original play.
While the kit was primarily intended to fit most Duncan Hard Core yo-yos and other models (see back of package), you could also use the parts in other manufactured yo-yos with some trial and error. Or, you could design your own yo-yo halves on a computer to fit the various parts of the kit, then have the yo-yo created using a 3D printer. For example, see the triangular "Onigiri".
