Most pen nibs do write when turned upside down. The ink flow is scarce, the line is thinner, and, more often than not, the feeling is far from being smooth. But the ink is there, ready to be used. So it was only natural that sooner or later someone would try to take benefit of it.
The two nibs on display today were designed in the seventies to write on both sides.
The Parker 180, well known in the West, was in the market between 1977 and 1985 in a number of styles. These pens had two possible nib combinations—either extra fine and medium or fine and broad.
The Platinum PKW-5000 pocket pen predates the Parker 180 in some years. The nib combination in this case is extra fine and fine, and comes in a formal black pocket pen style, as many other pens in Japan during the 1970s.
Close-up of the tips. The fine line of the Platinum, on the right, looks a lot thinner than that of Parker's.
However, the origin of this type of nib lies in the 1966 Sheaffer’s model called Stylist.
My thanks to Kinno-san.