MQ137 Ammonia sensor - Pins not labeled
According to figure 2 in the referenced datasheet, it doesn't matter how you fit the sensor in the socket. It fits in only two positions in the socket, 180 degrees rotated. Pins A and B are interchangeable according to the circuit on the right and are both double according to the top-middle drawing where the heater pins (H) are the middle ones of the triplets.