I think you guys saying it was the Giant TCR that was the first one are correct. By that time sloping top tubes and aheadsets were standard for mountain bikes, and it really was only a matter of time before some company tried the same format for with road bikes. Since Giant mostly made mountain bikes at the time, it makes sense. I'm slightly surprised the UCI didn't ban it, but it is still a double triangle.
WRT the OP's Sekine, IIRC a couple manufacturers used sloping top tubes in the smallest sizes purely as a practical way to be able to fit 700c wheels on a 19" frame without excessively weakening the front of the bike. I believe that is the case here.
That Fred DeLong bike strikes me as oddly prescient. Obviously there were some other outliers, but these to me look less like the modern format, and more like a semi dropped top tube compromise between full 'ladies' bike and normal diamond frame.
Dropped top tube were also common during the original bike boom at the turn of the (20th) century. For that matter one might call American cruiser type bikes semi dropped top tube. These date from the 20s and 30s, through the 50s, and frankly into the present
Last edited by Salamandrine; 05-25-20 at 09:42 AM.