16mm dual mono
I think that “dual mono” refers to 2 separate mono tracks (allowing, for example, to select the language in a dual mono dolby digital broadcast), which is not the case here : it is exactly the same track on left and right channels.
("“Dual mono” indicates two completely separate, independent, unrelated channels" : https://www.sweetwater.com/insync/what-is-dual-mono-versus-stereo/)
So the best word to describe this 16mm sound is just “mono” without “dual”.