This pair of artifacts was part of a series of ancient tests on magic concerning space-time. The original use of these are now lost.
It appears as two large discs 10ft in diameter. One side of each disc contains the "portal", the other appears as a mere solid adamantine plate.
This portal connects the lines in space between the two discs. The portal does not appear as magical for any purposes and the line of effect/sight transmits perfectly across through the portal and out the other disc. Any object or effect that travels through one disc continues out the other disc at the corresponding position. This works regardless of where the portals are, even if they are on different planes.
The discs can be handled, moved and rotated without affecting the portals apart from moving them. Anything halfway through the portal when it is moved is pushed along with its motion to maintain it's position relative to the portal. This does not exert any force on the disc itself and will exert as much force as required to move the object. Immovable objects break at the portal when this happens.
At one point along the edge of both the discs is an attachment that serves to mark the connected position in the portal. An object entering the disc just under the attachment of one disc exits just under the attachment of the other disc.
The attachment has a dial with two settings: facing forward to the portal face of the disc and facing backwards to the back. A forward facing dial is the default and corresponds to it's normal operation described above. Turning the dial to the back causes the portal to invert all it's linked positions along the diameter that includes the attachment. Anything placed through the portal when the dials on the discs are in different settings is mirror imaged in the transit.