Some TiVo units of the same model number were made with different fan sizes.
The way to determine which fan you need is to visually inspect your fan. If your fan grill juts out from the back of your unit, you need a "Raised grill" fan. If your fan grill is flat/flush, you need a "Flat grill" fan. In the picture above, the op unit has flat
fan grill and the bottom has raised fan grill.