In case anyone else is still a bit unsure then the -40dBc relationship for the IMD3 terms then it is shown in the marked up image below. This level of (distortion) performance is predicted with the classic equation in post #18 when the mixer drive tones are at -5dBm each. The mixer has 7dB conversion loss so the plot below shows the two main (wanted) test tones leaving the mixer at -12dBm each in the analyser plot below. The two (unwanted) IMD3 tones are about 40dB below this level as predicted byt the equation
Also, I pasted together a schematic of the tenTec 1210 transverter from a German review of this transverter. You can see the transmit signal path from the input, through the attenuator and the 28-32MHz BPF, the mixer and then to the J310 JFET Q6.
I've circled PIN diode D8 in the input attenuator and also the L4/C13 trap in the 28-32MHz BPF and also the J310 JFET Q6.