For each Numbers document, I am seeing two files, one with the extension, one without. (Which one to use??! ) Thanks for directions Phil0124 on how to see if the system is set to show the extensions (I looked before and could not find it - it has been many years since this was of interest.) The box "Show all filename extensions" is unchecked on all my computers.
I have two computers on this network, one is a 2017 iMac (not really supported anymore) and having all sorts of weird issues. Apple engineers are working on fixing a collaboration issue (between my Mac Mini and my wife's laptop, both up to date systems). I suspect the old iMac is at the root of the problems and have purchased a new Macbook Pro and retiring the iMac.