As LCGuy points out quite strangely, the serial number is under the front left lip next to the contrast knob. That's how I KNOW which model it originally was. The seller includes a picture of it on the photobucket links and it was manufactured the 17th week of 1984, which means it was originally a Mac 128K.I have a couple Plusses (Plusses? Pli?) that was upgraded from either a 128 or 512. I've been too lazy to crack into it to see if there's any way to tell which one it was.
So you don't really have to crack a Mac open to know what it was originally as long as the serial number is present. What it has become is easily determined by looking at the rear bucket, checking the About Finder for RAM info and noting the FDD type.
I figured the consensus would be vandalism about the missing label, but you never know, especially about the first run of something.