9's Complement BCD Thumbwheel Switches


I'm helping a guy with a microcontroller project.

He's going to use some BCD thumbwheels to set a time delay. He
purchased some at a local surplus store for a buck each. But they
were 9's complement thumbwheels, not standard decimal-coded switches.

I'd never heard of 9's complement switches before, but from the truth
table he sent me, I found that decoding them was no problem. (See
below)

My question is this: what sort of equipment might have used these
switches, and what generation? I read about that system (and
complement arithmetic in general) on Wikipedia and surmise that they
might have fed a decimal adder, or somehting like that.

Has anyone ever encountered these, and on what sort of equipment?
What were they used for?

Thanks!

Tom

====================

Reference:

Decoding 9's complement thumbwheels

Truth table:

1 = closed switch
0 = open switch

Switch Setting 8 4 2 1 Complemented Decimal

0 0 1 1 0 9

1 0 1 1 1 8

2 1 0 0 0 7

3 1 0 0 1 6

4 1 0 1 0 5

5 1 0 1 1 4

6 1 1 0 0 3

7 1 1 0 1 2

8 1 1 1 0 1

9 1 1 1 1 0

Connect each switch common to ground, then connect the 8, 4, 2, and 1
switch terminals to the microcontroller using pullup resistors. (That
active-low arrangement automatically inverts, or complements, each
switch condition, completing the first step in the decoding process.)

Read each thumbwheel as a BCD nibble. Subtract that reading from 9 to
arrive at the decimal switch setting.

.