The BCs are part of the San Bernardino County Fire Chiefs numbering plan (meaning all departments in SBCo subscribe to this plan).
Chiefs in SBCoFD all start with 1+ then have other numbers attached. That's about all the rhyme or reason but I'll drill down further...
Normal assignments are as follows with one person on each shift. Some of these are unfilled vacancies but I don't have that breakdown.
BC118, 119, 120 (Battalion 1 - Upland)
BC121, 122, 123 (Battalion 2 - Fontana)
BC124, 125, 126 (Battalion 3 - San Bernardino South)
BC127, 128, 129 (Battalion 4 - San Bernardino North)
BC130, 131, 132 (Battalion 5 - Mountain county fire areas)
there is no Batt 6
BC137, 138, 139 (Battalion 7 - Yucca Valley/29 Palms/Joshua Tree)
BC140, 141, 142 (Battalion 8 - Phelan/Hesperia/Lucerne Valley)
there is no Batt 9
BC147, 148, 149 (Battalion 10 - Adelanto/Helendale/Trona/Baker)
Now that you see the order and logic, here's the messy part: When they work overtime and cover someone's vacation leave, they keep their normal number. So, on the day you're referring to BC126 was out of battalion covering Batt 10 and went to the Hinkley call.