[cybmole]

OK - but initially I did not even visit that screen!. I followed my normal procedure of clicking convert books on the menu bar then setting the source & target formats. I was not aware of any need to also go set things on an additional screen.
To be fair, I confirm that the current default on that 2nd screen is mobi - old, so maybe I had changed my preference to mobi - new at some time in the past, or maybe some past calibre update affected it.

I still am not getting the deference between mobi-new and AZW

drifting off topic, now we could have gone down the amazon family library route in order to share a few books for a few days, but lI balk at this in aamzon T&C
"..In order to share content, both account holders need to authorise each other to use credit cards associated with their Amazon accounts for purchases on Amazon. This will not affect either of their current payment settings, but each adult will be able to copy the credit cards of the other account to his or her Amazon account and use them for purchases with Amazon."
not because I don't trust my daughter but because I don't get why she needs access to my payment method just to read stuff I've already bought. . If her kindle got nicked from hospital while she was busy giving birth, then having granted it unlimited access to shop on my card seems a pointless security risk. & thefts of small but valuable gadgets from insecure hospital wards does happen