And, just so you know, the LAPD never had (at least back during the show) a 1A12 unit.
That's right that LAPD has never had a
regular patrol unit designated as "1A12," but for a while beginning in April 2003 they did start deploying a 1A12 as an "emergency response" unit in Central Division, staffed by high-performing officers as a sort of recognition for their work, at the discretion of the division commander.
The first evening this unit was assigned, Kent McCord (TV "Officer Jim Reed") attended Central Div's PM watch roll call, and also stopped by the Metropolitan Dispatch Center and met the woman who was dispatching on Central that night.
See
"LAPD's Central Area to Activate 1-Adam-12 Patrol car"
Oh, the person who came up with the idea was the Captain of Central Div at the time, and is now Chief of Police, Charlie Beck.
Nor were they referred to on the air as ' 1 Adam 5', they were always said as ' 1A3', no phonetics.
Actually there's no hard and fast rule about using just the letters rather than the phonetics or vice versa. Simply by tradition, some unit types are almost always ID'd phonetically (e.g. "David, Edward, Henry, Paul, Queen, Victor" units), some only by the letter ("B, R, Y" units), and some are spoken either way ("A/Adam, K/King, M/Mary, T/Tom, U/Union, W/William, X/Xray, Z/Zebra" units). Their MANY Radio Unit designations are listed in the LAPD manual here, in section 4/110
http://www.lapdonline.org/lapd_manual/volume_4.htm#110