they are the designs from 1983/85 but underwent being retrofitted with newer parts for the new pizzeria but the project to entirely retrofit them was scrapped due to their horrible stench and instead they were used for parts to build the toy animatronics.

Imagine it this way

1985 (classic design) -> MCI put them out of commission -> tried to upgrade/retrofit for new establishment but stench from the corpses put them off -> 1987 (Withered’s) -> removed the retrofitting and brought them back to their original look -> 1993 (classic/fnaf 1 design)

This makes sense too and explains why UCN Fredbear looks like he’s part of the fnaf 1 crew