Jake, apologies for the lack of a reply. Every day I think about replying and then wonder where to start. There are two, possibly three stories which have blended over the centuries.
You call it Shinkiro which is correct; 蜃気楼 (the word literally means ‘clam spirit towers’). The word must have come from the seller. And we can see the whole scene emanating as a breath or spirit or dream from the clam on both sides of your lovely Kozuka.
So you could ask where the story of Urashima Taro started, or you could ask where does the kingdom of the Ryugujo Castle exist? You could say it is an unattainable paradise which can occasionally be seen over the horizon, a mirage or the dream of a giant clam.
And then there is the food element in the story, a palace with gorgeous seafood (clams) and the sexual element as the clam representing female genitalia, the delicate Shunga element hovers in the background.
So in the children’s story, as a reward for saving the turtle’s life, Taro the young male is taken to the towered palace of the sea princess and entertained variously and endlessly until he becomes homesick.
箱 = Hako or box, case.
The 手箱 tebako vanity box or treasure/jewel box 玉手箱 tamatebako is given him as an amulet on condition he never opens it. And the rest is history.
But mirages continued to appear, especially along the Japan Sea coast of Japan, with Korean or Chinese towers, and the legend of the shimmering towers in the spirit of the giant clam would reappear everytime someone used the old word Shinkiro to describe it, and children would wonder anew.
So we see Netsuke clams with a palace carved inside them, or Kozuka like yours, for that is where the Ryugu Jo where Urashima Taro once visited continues to exist.