BrickLink - Part 4760c00pb06 : LEGO Electric 9V Battery Box Small Without Battery Cover with Unitron Silver / Green Pattern on Both Sides (Stickers) - Set 6991 [Electric, Battery Box] - BrickLink Reference Catalog
LEGO Space Unitron 6991 Monorail Transport Base with Original Box - Works, Clean 42884069919 | eBay
LEGO 6991 Unitron Monorail Transport Base | BrickEconomy
6991-1: Monorail Transport Base (building steps) from BrickLink Studio
Monorail Transport Base : Set 6991-1 | BrickLink
Lego 6991 - Monorail Transport Base has been rebuilt! : r/lego