I was interested in the master key idea since we now have four Rads. When we bought our last bike I asked Rad if they could send me a bike with the same key code as one of our other bikes but they can't do that. So I got one of the Honda blanks as mentioned earlier in this thread. Here is what I found:
The blank does not work in our two 2016 Rad City bikes and none of our other keys will work in these locks either. (These batteries have a simple key locks, the key does not switch the bike on or off.)
The blank works in the 2019 and 2021 Rad City Step-Thru bikes.
The 2019 bike uses a F07 key and the Blank, F02, F04, and F06 keys work in it.
The 2021 bike uses a F02 key and the Blank, and F06 key works in this battery.
All the keys have 6 cuts and each key has a different cut.

I'm not too concerned with posting my key numbers since they are not secure to start with. It is more of a pain in the butt juggling all the different keys. I would like to see if I can get a key cut that has F04 on one side and the F06 on the other side then one key would work on all the batteries.