I was just inside my SWG control box a few days ago to solder a new inrush limiter on the board. My board is also an r1.40 release.
It has a jumper on the main board in the upper left quadrant. The jumper is hidden under the daughter board that has the display and control switches.
To check, turn off the power to the control box (display goes blank). Disconnect the cell cable and remove the front panel screws on the left and right sides. Use the finger notch at the top to pull the cover forward and up to remove it. You may be able to see the jumper pins. If it is the same as mine, no jumper=T5, jumper in place=T15. The jumper is clearly marked with the T5/T15 designation on the board.
If you need to remove the daughter board to see or to place a jumper to switch to T15, the daughter board is held in place by four plastic posts, one at each corner of the board, and a 7 pin riser that connects the daughter board to the main board. To remove the daughter board you pull it (gently) toward you while squeezing the four plastic connectors at each corner. Wiggle it a bit and squeeze and each post connector will come loose. Then pull straight forward to unseat the board from the riser pins. Replace it by reversing the steps above. Once you have it off you will see how the daughter seats on the riser pins. It's pretty straightforward.
Hopefully it's just a matter of placing a jumper, and may explain why the previous owner never used it.