Well the receivers are not identical. The good one is a "portable" version with a build-in battery, I bought this one 1,5 years ago. The bad one is a "standard" receiver and I bought this one half a year ago.
Since they are not identical designs, the performance may indeed vary. Lawmate sometimes does odd things that really make me wonder what they are thinking. For example, your old portable seems to have a video level pot in it. The new one has a location for the pot. But they stuff a fixed resistor. It makes me want to slap them silly.
What is interesting is that, while the video looks good, the image from the old good receiver looks "better". It looks brighter while the video levels are the same.
Are these the same two receivers we discussed in the recent Oracle troubleshooting? If so, then the brightness difference is probably due to a mismatch in the luminance levels. Although you had "1V" video levels, the syncs were stunted and the video luminance was increased to make up the shortfall difference. So, in your comparison, instead of looking at the overall video waveform, compare just the active video region (bottom of black to top of white); Ignore the syncs. This will give you a better method of determining why the two images do not have the same brightness.
Could it be that also the BW of the video is reduced and that this affects the colors?
Color intensity/purity can indeed be related to video bandwidth. But you would definitely notice a difference in perceived image resolution.
On the other side of the mother board there is a small pot. What would that be for?
The big board has two pots. The mid-right one seems to be the video level pot. On both boards the other one seems to be a variable cap for a local PLL. It is near the two ceramic filters and may indeed have something to do with the sub-carrier performance. If you tweak it, be prepared to tweak it back.
By the way, do the two ceramic filters have the same part numbers on both Rx's? These are the three pin orange-brown colored parts.
It is also on the mother board that the switches are to select one of the 8 channels. How does that work?
The DIP switches connect to a microcontroller that I2C communicates the PLL divider codes to the Rx module.
Would it be a good idea to test with another channel?
Yes. Try all the channels.