this is an insert for the Lithograph Frame by hugo.workshop

https://makerworld.com/en/models/1036463-lithophane-frame-for-bambu-led-cmyk-b-w?from=search#profileId-1020379

it fits in place of the flat lithograph and allows use of curved lithographs. They insert from the front and can be replaced easily. I did this to allow the frame to display seasonal work. 

This is the beautiful curved lithograph by hunterc76 that inspired the adapter. 

https://makerworld.com/en/models/2020473-christmas-nativity-lithophane?replyId=1709077&replyType=3#profileId-2177333