Opus was a replacement motherboard used when systems were sent in for a rrod fix. God knows how many are out there.
A couple of years ago I had a Hong Kong Famicom that looked identical to normal Jap Famicoms, but it had an extra switch at the back by the RF channel selector which said "Normal/slow". From what I gather it was designed exclusively for the HK market which uses PAL (I think), hence the switch. No idea if it was that rare, though. Some more info about them here: http://www.famicomworld.com/forum/index.php?topic=4138.0