A more ‘recent’ post compared to others here: an article by CPUShack on the Intel Inboard 386/AT, a plug-in board for the PC-XT/AT that ‘upgrades’ the processor from the 8086/80286 to the 80386. Some cards also featured faster RAM and even (gasp) cache memory.
With the release of the 32-bit Intel 386 processor in 1986, owners of IBM PC/XT and AT type systems (8088 and 80286 systems) were left a bit in the dust. This was a concern (or opportunity) for Intel as well. They designed an upgrade solution at the same time as the 386, to be able to be used in the now obsolete computers. This was the Intel InBoard 386 series of upgrade cards.