Here's another two cents. "Rodent" is absolutely correct There were three Dodge D300 cab and truck chassis owned by the studio and used in the series and T.V. movies. Squad 10 in the pilot was a 1970 Dodge that was on loan to Universal from the LAcoFD. The first Squad 51 was a 1972 D300 and it's common knowledge today that the Fabrication Shop at Universal built this chassis into a faithful replica of a LAcoFD rescue squad. This truck lasted two seasons and prior to season three the studio acquired a 1973 D300 truck. The '72 was stripped of all useable parts including the rear compartment box. The front grill was also swapped to the '73 truck so the stock footage of the running squad shot in the first two seasons could be used again. Watch season one and you'll see why. The '73 truck was virtually identical to its older brethren with the exception of the two "D" ring towing lugs bolted to the diamond checker plate rear bumper and the Dodge letters on the front of the hood being painted over with the exterior color. Once again, this truck survived two seasons and was replaced by a '74 D300. And again the older truck was stripped to help build the next squad. This is the vehicle that currently resides in the LA County Fire Museum. Decode the V.I.N. of this truck and it will indeed reveal that it's a 1974 Dodge D300 1 & 1/2 truck cab and chassis. (The excellent book "Emergency: Behind the Scenes" incorrectly states that the museum truck is a 1972 model, yet they provide the V.I.N. that proves otherwise). The '74 is discernible from the previous two squads by the rear external box light and the black vinyl upholstery trim inside the two cab doors. The earlier trucks had bare metal doors painted to match the exterior with beige coloured arm rests. An interesting and ironic twist to the Dodge rescue squad truck story. When Universal actually began filming the pilot movie, the LAcoFD had stopped purchasing the Dodge truck to supplement its fleet and began using Ford trucks. When Universal denoted their Squad truck to the County, it was the "newest" Dodge truck in the department. The latest Dodge trucks owned by the County were 1970 models.
On a sidebar, back in 1980 when I was twenty years old, my buddy and I spent a week in Los Angeles and we toured the Universal lot. I happened to spy a studio employee who was wearing a "Squad 51" t-shirt. Being the fan that I was I had no apprehensions about approaching him and asking if he had worked the series and did he ever meet the cast. Regrettably the answer was no, but he worked in the Fabrication Shop and was part of the crew that built the third squad. I asked what happened to the other two trucks and he said after they were stripped they had no real value and nobody knew where they ended up.