I’ve got some quotes from Total Film #317 saying, "In order to recreate Egon’s Proton Pack, Harold Ramis’ original prop was Lidar-scanned, rebuilt “down to the millimetre” and aged to give it “the kind of battle scars you would think it would have by now”. And also in SFX Magazine #345 and the Afterlife Art of book, it’s mentioned they got “the original proton packs from Sony’s archives in order to scan them for our own equipment.” Ah, I’ll have to go look for some photos of that, I think there are. Now this is bugging me.
edit: Can only see a bit of the pack: