There's a long story about that.They weren't "revenue officers",they were meant to prevent fireworks (illegal in MA,legal in NH) being brought over the line.That was some time ago...very messy affair! More recently ( a year or two ago) Massachusetts decided to demand of a tire store chain that they report the installation of any tires on cars with Mass plates done at any of their NH facilities.The Governor of NH told Massachusetts to go bleep itself and the NH legislature quickly passed a law forbidding any such act.
It was beautiful to watch!
Some years ago Pennsylvania had people noting down license plates of PA cars stopping at a NJ liquor store just across the bridge, and stopping the cars as they came back into PA with NJ booze.