Both were running visible to all. Worked using visible only to 2017+. Thank you WoodSTokk.
P.S. Clarification. When I first encountered the problem, one machine was running 2018.3.2 and the other
2019.2.1. Selecting "only 2017+" on the 2018.3.2 machine made both planes visible to one another.
With both machines running 2019.2.1 no problem exists with mutual visibility; but I also note no "visible to all" selection remains available in 2019.1.2.
All this was a bit confusing to me, as I was using the Navion-an older plane, and was unsure what "visible only to 2017+ " versus "visible to all" meant.
If I continue using 2019.2.1, does this mean I won't see planes running on older versions unless their user has independently selected "only 2017+" in their MP ? And how many user planes running on earlier versions will ever be visible if a "2017+" selection is nonexistent in their MP ?

As usual.