So I have a few of these at the house and thought some folks might find it interesting. '81: NEC c4558c, dark green caps, huge electrolytic cap above the chip, "510A" on the board '85: JRC 4558DD, bright green caps, smaller electrolytic cap above the chip, 510B on the board Recent MIT (sorry for the bad pic) JRC 4558DD, silver caps. Some other minor difference (like the size of those four ceramic caps). They all sound like an SD-1. My initial impression was that the '81 was brighter with less saturation, the '85 was warmer with more saturation, and the MIT was somewhere in the middle. But after playing for a bit and switching back and forth, I'm not sure I actually can tell them apart.