Dave Farquhar at The Silicon Underground compares the two versions of the original IBM PC 5150, pegged to the October 27, 1982 date on the Revision B BIOS. The fronts look identical. The back tells you which one you have: Revision B is marked with a B in a circle and has positions for five screws, while Revision A has two. The Revision A also shipped with a black power supply instead of raw metal.

Revision A, sold from August 1981 to March 1983, is the scarcer and more collectible one. It is also the more limited one. Its early BIOS tops out at 544K rather than 640K, ignores option ROMs on EGA, VGA and hard drive controllers, and the machines often came with single sided floppy drives. Revision B takes a full 640K and runs anything short of 286 software.

Farquhar estimates roughly 600,000 Revision A units against about 2.7 million Revision B, out of 3.3 million 5150s total.…