Apple offered an upgrade kit for all 128k & 512k Macs that allowed a service tech to upgrade their earlier Macs with a Mac Plus motherboard and a back that said "Macintosh Plus". The front faceplate would stay the same, obviously, but the old back would be tossed and the new back put on. Because so many original 128k Macs were upgraded this way the original, unmolested Mac 128k sells for a pretty penny. Either way it makes your original 128k or 512k a lot more useable, just not as collectable.
Hope that helps!
I have kind of the same, original 128k (rev.2) case, original board but with 1Mb expansion card with SCSI connector as well, with Mac Plus ROM chips and 800k drive, making it an Plus in a 128k case.