Macintosh History from the 1980s
From a report on the MacWorld Expo in 1989: One of the most important developments
> Accelerator cards for the Mac, “believe it or not”. This was presumably one of the first time such cards _could_ be developed. Two prerequisites had been met: first, the debut of the “Open Mac” architecture of the Macintosh II a few years previously (March 1987), and secondly a processor advancement (from the 68020 that shipped in the II, to the 68030 in the IIx and SE/30.) Of course, previous closed Macs such as the Plus could be upgraded with special clip-on chips which boosted clock speeds, but 1989 was the first year that accelerators could be introduced based on the Apple-approved NuBus boards.
Now we’ve come full circle and uprading any recent Mac is pretty much impossible once again
This is a fun site. If nothing else, vist it for the design.