In case you don't get a reply from someone with specific knowledge about these parts, I can do some guesses. Note that I am making a lot of assumptions here.
From what I can tell from the manual, the SM-IR-2 board also uses common positive and switched negative. The outputs have no current regulation, so resistors are necessary.
https://www.beier-electronic.de/mode...IR-16-2_GB.pdf
If the carson style led board is the same you ask about in the other thread, I assume it's this one:
https://www.rc4truck.de/Fuer-Tamiya-...volt::816.html
I see they come in 12v, 7.2v and 4.8v versions. I assume the difference between these are the onboard resistors, because that is the only reason I can see to have different versions. You should probably confirm the existence of resistors visually. Looks like one side has LEDs and the resistors would be on the backside that I can't see on the pictures. You might be able to read the markings on them to find the resistance values, or you could measure them with a multimeter.
We can make some estimates for the minimum value of the resistors used though:
Red and orange LEDs generally have a voltage drop of between 1.8 and 2.1 volt. White are around 3.2 volt. A common safe maximum current is 20mA.
You ask in the other thread about connecting the 7.2v version to 12v. If we assume that each LED has its own resistor, we can estimate the values.
7.2v - 1.8v (red) = 5.4v. 5.4v / 0.020A = 270 ohm
7.2v - 3v (white = 9v. 9v / 0.020A = 210 ohm
The same calculation for the 12v version gives 560 and 450 ohm as resistor values.
If two red LEDs are connected in series, the voltage drop would be higher and the resistor a lower value. Using the higher value will be safe.
Using these values we can take a guess at the worst case for connecting the 7.2v version to 12v:
12v - 1.8v = 10.2v 10.2v / 270 ohm = 0.037A.
37mA is almost twice the rated current.
However, we can see that by adding a 330 ohm resistor to each negative wire, the combined resistance would be 210+330 = 510 and 270+330 = 600 which are higher values than 450 and 510 ohm respectively.
In conclusion, adding at minimum a 330 ohm or preferably a 470 ohm (or even higher) resistor to each negative wire on the smd LED boards should in theory let you safely connect them to the SM-IR-2 board running from a 3s battery. Higher value resistors are safer. 1000+ ohm should still be bright enough.
Good luck! I hope you get a better answer from someone who actually has experience with your actual parts.