Place the coin you found into the coin slow under key 6. The coin slot will morph into a button. Press the button to start the piano puzzle. Copy the notes and get them correct three times to unlock the crossed metal handles.
Add these metal handles to the cross shaped keys on the section of the music box that appeared previously. You must turn the keys to make holes and then slide the pieces into the holes. Turn top left, slide, turn bottom right twice, slide, turn bottom right, slide. Finally turn top left and slide.
The eyeglass will appear and show a cube. Get the red gemstone on a metal plate. We are nearing the end of The Room Epilogue level now.
The gemstone puzzle solution in The Room epilogue. Slide all 4 lower piece to the right. 2 Pieces down, 1 piece right, greed down and yellow all the way to the left. Three pieces up and to the right. Blue left and into the hole, green down to blue. 1 piece left, 1 up, 1 right and green right. Finally, move yellow down between blue and green. Move all other pieces up to make space for the colored gemstones.
Look at the back of the gemstone board and you can pick out which colored gem or crystal is in the hole. Slide them each into the hole one after another to get the blue gem, green gem and yellow gem. They might be diamonds and remember you already have the red gemstone.