a 31st birthday present for a cameraman
All the pieces of a dismantled 8mm camera, as a whole, make up the number 31.
The dismantled pieces are also clustered in different sets of 31 pieces with the same characteristic(s). The residue, the camera's empty body, contains a list with the characteristics for each set of 31 pieces.
Lastly, the camera's manual is placed at the back of the main board and is 31 pages long.