Apple 815-0726, 815-0938, 815-0971.
Battery Door for the Macintosh 128K, 512K and Plus. Early covers have a large battery symbol in the middle of the cover. Later ones (beige and platinum) have a small battery symbol towards the top and also have two foam pads glued on to the back.
A) Early beige (on left in photos)
B) Late beige
Note that some may show discoloration just like the cases, especially on the Platinum models.