Fired it up and compared it to it's younger brother, the green label 11-89 MIJ.
Tonally, they are close enough to write off as general circuit tolerance.
Visually, there are many differences. The 80 is lighter purple, the enclosure does have some different specs (the footpad part is a tough thinner, the battery compartment has 2 access holes, different components, different circuit boards, etc...) What's interesting is that the 80 has a round recess on the inside of the footpad where it contacts the switch, where the 89 has a protrusion. Despite having several late 70s/early 80s Boss with this same recess, I have never noticed it before. Also, the later revision where BOSS made the battery compartment fully sealed up is a good move. I imagine this removed the possibly of a leaky battery ruining the circuit board.
Some comparison pics for fun and reference... Early MIJ 80 on the left, late MIJ 89 on the right.
Here it is on the 80s board with it's other silver screw family.