Hmmmm. I don't know. However if it were on top of the downtube I would suggest it was a direct mount for Suntour Symmetric shifters (they were also available as a clamp on). You say there is just one hole? Could it be for some screw on cable stop? Or shifters? How about a photo?
Hey look at post
#8 in this link to this forum. Look closely at the cable stops. They look like they're mounted at the bottom of the down tube and I see no clamp.
http://www.bikeforums.net/classic-vi...on-thread.html