I’d be a little careful about calling the level shifting circuit “proper” – the resistors in the circuit are 1K – 1K to +5 and +3V3. That’s a pretty low impedance line, especially considering that the Arduino Pro has 1K resistors on the TX and RX lines (why is this by the way???) What bothers me is that the stand-alone level shifter (BOB-08745) has 10K resistors. I substituted 10K (Ask me how fun soldering was with only a regular iron and tweezers) and it at least works now with the Arduino Pro, but it barely pulls “LOW” to 1V
I should say it works FINE with a Leonardo (and probably the Uno too) because they lack the resistors on RX and TX.