This particular saya appears to be a typical shirasaya at first glance but upon closer inspection there are some things apparent that raise more than a few questions

The ireko saya is quite unique in that it is really a saya within a saya.  The Ireko saya seems to be a product of the Edo Jidai and is rather rare.  When found, they are almost always on koshirae of the highest quality.  The Ireko saya allows easy cleaning of the interior  as it is removable and since nuri can be quite expensive, the irekosaya very cost effective. 

There are really are no rules for the mechanism of ireko saya and is limited only to the ingenuity and skill of the sayashi.

This particular mechanism involves the removal of a retaining tab that physically locks the inner saya in place and is very unique. All pieces are made utilizing blond horn.   Blond horn was relatively uncommon and highly regarded.

A close up

Inner saya is unlocked and removed.

Inner Saya is removed and swivels open on a specially prepared hinge.

The small horn at the koiguchi act as a catch for the retaining tab to fit into and lies below the plane of the surrounding wood.

Hinge disengages for easy cleaning.