This is a difficult, and time consuming question to answer, particularly as we do not have the car in front of us, or a picture of the part.
I had a look at the workshop manual, and as best as I can see, those Lacetti's fitted with rear disc brakes have the type where the disc is also a drum, for provision of the shoes for the handbrake. As I was working doing delivery driving yesterday, I stopped and had a look at a parked Lacetti and that seems to be the case.
The trouble is that the workshop manual shows an exploded view of the rear brakes, but only shows the backing plate for the purely drum version that was sold in some markets and has the wheel cylinder attached to it, the exploded view of the disc version does not show the backing plate, or part number, but it can be seen in some of the other braking diagrams-but no details.
No reason is given as to why these parts need to be replaced, however I would guess at it being due to corrosion. As Victor has previously pointed out, sometimes people come on here and pretty much "threaten" to scrap a car if we cannot help with parts supply. It is different at my home, If a simple part like this was NLA, and an MOT depended on it, I would use the remains of the original, and a cereal packet, to make a pattern, and then make a new one from sheet metal, even if it did take all afternoon!
There are a small number of these cars being broken on eBay, but who knows what the state of this part would be on those cars, and who would be willing to remove them for a reasonable price?