Blimey... good luck with that.... you need a super-rare internal wedge seatpost. I know a few old Schwinns used the same system, it's exactly the same as the old wedge type stem, but in seatpost form!!
I don't fancy your chances of digging one of those up.... so you could try the old Vitus trick.... drill into the back of the lug, tap it, and screw a bolt in against the seatpost... the only problem will be if there's enough material on the lug to hold the bolt without stripping the threads.