What Atari system does this 1980s PCB go to?

It looks like this would have been in an Atari CX-2600A On page 1-6 of this field service manual, there is this image with the caption "2600A Board Layout (Revs 1-13)":

Since that is a revision 8 board, it should have been in one of these systems (and also likely to be nearly identical to revisions 1-7 and 9-13). You can tell it was for a four switch console by counting the number of "S20X" components. These would have been the actual switches on the outside of the console, and you can see there are four:

• S201: Power (far left of board)
• S202: Game Reset (far right)
• S203: Game Selector (middle right)
• S204: T.V. Type/Color Selector (middle left)

The manual also states:

The major difference between the newer single board VCS (2600A) and the original VCS (2600) is that all of the components formerly on the switchboard are now located on the motherboard (See Figure 1-5). This includes the player control function switches (Power ON/OFF, COLOR/SW, GAME SELECT and GAME RESET), RF modulator and power supply circuitry. The single board design eliminates the need for the ribbon cable, which connected the switchboard to the motherboard on the 2600 vcs.

Notice how it makes remarks to the switches being on the motherboard now instead of a separate switch board.

Probably the biggest giveaway though is this parts list in the manual:

The "CO15519" number is the part number for the motherboard, and at the top of page it states "CX2600A Domestic (M/N) VCS".