My uncle has owned two R6s, the last of them, however, over 20 years ago.
His first one - which I just barely remember - was a '75 R6 TL which he bought new. It was the facelifted model with square headlights, plastic grille, bigger bumpers and bigger rear lights. He sold it two years later (I was four by then...) to fund a new '77 R12 TS.
In the late '70s he bought a used pre-facelift R6 TL (maybe a '71?) to use as a "work horse". These earlier cars had round headlights and aluminium grille.
The R6 uses the same chassis as the R4. Therefore the same weakness is also its Akhilles' heel: the rear suspension mounting points.
Whereas the R4 has bolted front and rear wings plus front inner wings, the R6 is more "normal" in that only its front wings are bolted. Front inner wings are welded to the bulkhead and there's also a solid front panel welded between the fronts of the inner wings (i.e. the panel is not part of the bonnet like in R4). The rear quarter panel is one-piece from roof to sill, there's no separate rear wing (and thus no visible seam in the middle like in R4).
Besides the chassis rotting away, I remember that my uncle had to weld the back corners of the rear quarter panels where mud builds up. In an R4 this is a bit easier as the rear wings can easily be bolted off and changed for new ones, but as this area is one-piece in the R6, a patch must be made and welded in.
From what I remember, the R6 had a much better heating and ventilation than the R4. This is no surprise, as the heater matrix is situated under the windscreen and is almost as wide as the 'screen (at least in cars destined to Nordic countries). There are also two vent-outs in the D-posts, so it's a genuine "flow-through" cabin, whereas the R4 is like blowing air to a bottle from where it can't escape.
Equipment- and interior-wise, the R6 feels much more like a "proper" car in that it has wind-up windows front and rear, padded dashboard and thicker seats with adjustable backrests front (at least for the TL).
For me as a little boy, travelling in the 1108cc TL with its 47 bhp also felt like travelling in a rocket, especially when compared to my dad's 845cc and 27 bhp R4! :lol: