On their last rescue, Squad 51 is trying to get out of a brush fire area with a patient. When they approach a wall of fire, Johnny requests a water drop from a nearby helicopter. Copter 2 responds to help them, but the back of the tail fuselage says 14, Indicating that it's Copter 14.
In the opening scenes where the squad is pilling out of the station it shows the normal unit then on the street shot it is an older unit red bumper/different grill, possibly 1968-1970? then goes back to the normal one, even a view from inside the cab shows hood Louvre's and body-lines from the 60's model.
On the initial brush fire call, the station PA system sounds a single SCU tone, the one for station 51. The dispatcher, however, reads off a string of units assigned to the fire, without any tones sounding for those units.
Roy is departing the sister's house with Emma in the cab of the truck. John and Winifred, in a Stokes stretcher, are in the bed of the truck. As they are departing Roy turns on the lights and siren. Directly exposed to the siren John and Winifred would be deaf by the time they got off the mountain.