With Diana there, this was some US-British event, so they probably had each dance couple as an American paired with a Brit (like Clint & Diana) ...
so, the guy dancing with Nancy Reagan is probably some British guy, not Tom Selleck ...
Sure looks like him though. Thank you.
It is him indeed... His turn to dance with her...
https://i.pinimg.com/736x/71/fc/0b/71fc0bb3264a7adcd5299ca5a355ce6a.jpg
Look at who else was at the same event...
THAT IS TOM SELLECK,
There is only ONE Magnum!!!