The 40mm band works with the 38mm; the 44mm band works with the 42mm
38mm S/M = 40mm S/M Size: Suitable for 5.1"-7.1" (130mm-180mm) wrist;
38mm M/L = 40mm M/L Size: Suitable for 5.9"-7.9" (150mm-200mm) wrist;
42mm S/M = 44mm S/M Size: Suitable for 5.5"-7.5" (140mm-190mm) wrist;
42mm M/L = 44mm M/L Size: Suitable for 6.3"-8.3" (160mm-210mm) wrist.