Now, the output of my toy SG is connected directly to IC-7410 through a homebrew step attenuator.
The graph corresponds to three different positions of the attenuator.
This is with the RF attenuator of IC-7410 on and off, respectively.
It seems that the output of the SG jumps around -34 dB. This is not due to the receiver, because it always happens with the same output level of the SG.