It's a "shell wrap", the celluloid handles are wrapped around steel handle scales with bolsters extending out at each end. Imperial made a wide variety of them. Celluloid will shrink over time, which it appears the handle on your knife is doing (the gaps between the ends of the wrap and the ridge of the bolsters).
I'd be inclined to believe the handles are original. In order for someone other than Imperial to put those handles on-
1. They would need the celluloid. How many people have access to that, or just have it lying around.
2. They would need to pry the tabs back that hold the metal handle scales on at one end, and then bend them back to re-attach them, which is no small task as it's easy to break those tabs. Those knives were not made to be taken apart and reassembled.
3. And they would need to re-install the pins and shield. Both the pins and shield look clean, so again, I'm inclined to believe the knife is factory original.
Replacing the wrap handles seems like a lot of work for such an inexpensive knife.
Based on the tang stamp, that knife could have been made as recently as 1988, which could explain why the wraps have not deteriorated more.