This sale is for a Mamiya RB67 / RZ67 Eye Correction Lens -0.5 Dioptre for the standard RB and RZ type L2 and PD prisms, and the RZ67 Pro and Pro II AE prisms, complete with a box (but note that it is the wrong box).
This dioptre simply clips into the prism viewfinder. This dioptre is very clean with absolutely minimal signs of use.