I got curious again about the trucks used in this series. I could have sworn I saw a gold hood ornament on one of them. So, I went back and watched again. I DID see a gold emblem in a few shots. That makes it a '73. But, the VIT didn't come out till '74. So, it is possible that a '73 was "upgraded" to '74 status and used alongside a real '74.
I also found a bunch of people who had posted information about the differences between the '74 and '75 models. And, it does appear there were quite a few trucks used over the course of the show.
The link below has some of the best information I have found. There were a couple more that had interesting facts, like different locations of badges and antennas and different air intake piping between the different years.
Looks like there were 2 '74s, a '73, and a '75 used routinely. There was also the '73 in the pilot episode (quite different) and a '76 used in 1 episode (different color).
Some of the information I found comes from what appear to be credible sources. And, some of those guys really know their KWs. So, I am inclined to believe them (especially Sagehorn!). One even gave the VINs of the 2 '74s.
For what it's worth...
https://www.imcdb.org/v042128.html