I've been driving the 280SEL this week, as my 300D is in the shop, and the 280 does have the triangle under the trunk. Of course, the 280SEL is the European version, and it spent its first 50,000 miles in Germany. My 300SD did not have the triangle, but it did have the mounting areas. I'm relatively sure that all W126 sedans will at least have the mounting area.
1976 300D (W115) - 330K and still going (sort of)
1991 300D 2.5 Turbo - Sold at 221K
1983 280SEL - Sold at 206K
1981 300SD - Sold at 232K