"The AA version is a single-port (CH0) version, i.e. Antenna Analyzer. The CH version is a dual-port version - CH adds CH1, i.e. Vector Network Analyzer. Presumably anything without an AA or CH ID is a dual-port VNA version."
"My first NanoVNA had the AA firmware installed. Both ports are still activated in the AA firmware but by default the two available traces are set in the menu to CH0 for making reflection only measurements. For making transmission measurements, either trace can be set to CH1. By limiting the unit to two traces the memory savings was used for a larger font size."
W5DXP and hwalker, and Larry Rothman, thank you for this further clarification. Not having the option to inject a signal from one channel while listening to the other might be an issue if this was specifically required for testing and would certainly be a considration when choosing the firmware version. However, if one of the traces can indeed be assigned to CH1 for transmission measurements, then that is less of an issue. Two channels are sacrificed to get a bigger font. I guess it depends on whether the measurements being done will require more than two traces and whether one doesn't mind the smaller font. I will stick to the version I have for now.
BTW, I have now ordered the 50ohm load, female SMA through and other adapters as well as a fine touch stylus. These will take some time to arrive, but hopefully will make things easier to to complete the calibration properly without additional leads and adapters.
