The Tin Man
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Among the things the adult Number Five finds above the door lintel of his childhood bedroom, besides David's dessicated trumpet-vine flutes, "there is a 'broken' puzzle made of the bronzed viscera of some small animal."
Besides the teaching aspect of this (the young Number Five loves biology), what relevance does the puzzle play in FIFTH HEAD?
Think not so much bronzed entrails here, but metal-coated organs.
Because they're Mr. Million's, the robot tutor's.
We know this because he's put them above the lintel himself, with the confiscated flutes.
What do they represent?
Symbolically, several things.
The Wolfe clan's lost connection to the organic, the surrendering of their souls; i.e., they've become so obsessed with their quest (read the cloning-as-immortality-trope for not just its hubristic elements, but its Faustian) and used such dire means to achieve their ends, they've forfeited their humanity. And so like many families did in the old days, to help preserve and remind them of where they've been, but can't return--the "things lost" motif--they've bronzed their much more organic equivalent of baby shoes.
And why does Mr. Million put the bronzed viscera above the door?
Because he's the Tin Man from Oz. But unlike Baum's robotic woodsman, whose goal in life is to secure for himself a heart, Mr. Million--easily the most humane character in all of 5H--already has one, and so he can afford to let his broken backup model collect dust above the lintel for all the rest of time.