Compaq LTE Elite Maintenance And Service Manual page 288

The emitter and receiver are similar in appearance. To tell them apart
easily, note that the receiver frame has one small hole [1] and the emitter
frame has two small holes [2] (Figure 9-44). The single hole in the
receiver frame corresponds with a single infra-red component on the
receiver board while the two holes in the emitter frame correspond with two
LEDs on the emitter board.