From my memory of my 78 F1 the mirrors had a recess around the rear same as this Tanaka mirror which was on the bike.
I ordered an original replacement from DS which is different with no recess on the back plate
(Library Pictures)
Tanaka
DS original
Am I wrong in thinking the Tanaka was original to the bike both are same size 4.5" etc just differing backplates
Cheers
Keith Snr