Hi Peter
Looking at the picture you posted and at the Pagoda group, I think we can narrow this down.
The picture:
W114 because it has the double bumper, W115s only had a single bumper, USA spec as it has side markers and sealed headlamps. It's a series I vehicle as it has small bumpers, no windscreen/A-pillar deflector, the side mirror is on a stalk (doesn't look like a W116 mirror on the quarter vent unlike the sedan where the quarter vent was removed and the mirror positioned at the leading edge of the door) and looking at the inside, we see it has a column shifter and a steering wheel with a chrome horn ring. Thus, it is an original /8 and provided that it hasn't been re-sprayed, then it must be one of the /8 or Pagoda colours.
We know it is metallic, so it can only be one of three:
387 H Blue
396 G Medium blue
906 G Grey blue
We can exclude grey blue from the get go.
Looking at some websites, I would guess that it is 387 H Blue. If you take a look at these two adverts for blue cars for sale, the Blue approximates better than Medium blue:
Blue:
http://www.geevers.nl/en/inventory/mercedes-benz-280se-lage-grille-flachkuhler-cabriolet-802Medium blue:
http://www.geevers.nl/en/inventory/mercedes-benz-280sl-automatic-861Lapis blue is from the year 1980, /8 were long superseded by the W123 by then.
Hope this help, but you should ultimately choose a colour that 'speaks' to you!