very, very few lugs have a shelf for the top of the seat tube to rest against. The only good reason for it in my view is on stainless lugs where you would want to polish the top surface of the lug around the top of the seat tube. Most frame jigs now use a cone to hold the seat tube at the top so it's best to have a round tube up there to register against instead of the pointed seat lug.
Seat tubes are really thin at the top, that's why I advise people not to try to use a hacksaw to get a stuck seatpost out on a good frame.
Is the tubing sticker still on the frame? Do you know what it was made of? There might still be some Tange Prestige floating around if that is what it was made of.