This was made some time ago. I used it for testing (recently) and works as it should. The 2732 EMPROM contains both the original version of Tempest, and the new Tempest Tubes. Moving the switch enables or disables the highest address line (A11) on the 2732; this cuts the 2732 into two halves, with only one or the other half enabled at a time. So when the switch grounds pin 21, the first half of the chip (original Tempest) is used. When the switch ties pin 21 to +5, the second half (Tempest Tubes) is used.