Excellent. Did you ask them about BN too?That's true and a somehow strange deviation from the formerly used letter "A" (= "armiert" = reinforced; Armour); see e.g. my Trinovid 8x42 BA.
Because I had no clue I phoned with Leica in Wetzlar ten minutes ago - as always a pleasurable call. I was told that the "R" actually stood for "Rubber", maybe an effort to use a more internationalist nomenclature at that time. But the friendly respondent hinted at the sometimes not perfectly consistent decisions of marketing & design people...
I suggested that the "R" could perhaps also mean "robust" but he negated that, pointing that all Leica binoculars are robust, be them wrapped in leather, rubber or polyurethane.
Lee