A few years ago I picked up a old Plus to convert into
an aquarium (I know,
I know; but it was a fun idea). I had never quite seem a Plus like this
before. It had a Sticker to denote it was a plus. It did not have printing
or a platic label. It was simply a sticker.
Now, Plus was stolen (why people stole it was beyond me), but I am still
curious of the history that Mac may have had.
At one point there was an upgrade kit for the Mac 512 to the Mac+. I
think it included an exchange logic ('digital') board and a new back
cover (since the I/O sockets are totally different). You used the old
PSU/video ('analogue') board, CRT, drive and front panel.
I assume you also got a label to stick on the front to say it was now a
Mac+. THis sounde like what you had.
-tony