IN WITNESS WHEREOF the undersigned, duly authorized, have signed this Treaty. DONE in triplicate at the city of Moscow the fifth day of August, one thousand nine hundred and sixty-three. (33-34)

Nope, this isn't exactly an attention grabber, either. 

In a way, the closing sentences of the PTBT recall the opening line. They again bring together the three authoring nations (the U.S., the U.K., and the USSR) and, with the signatures of the authorized government representative for each nation, make the treaty the real deal. Those signatures, by the way, belong to U.S. Secretary of State Dean Rusk, British Foreign Secretary Alec Douglas-Home, and Soviet Foreign Minister Andrei Gromyko, and they made the original parties officially responsible for the treaty—as is implied by the opening statement. 

The closing lines are perhaps the most jargon-y of any part of the treaty, but "in witness whereof" and "done in triplicate" are standard legal terms. 

The former just means that the government representatives have signed the treaty in full agreement. The latter means that there are three copies of the original treaty, and each copy was signed by Rusk, Douglas-Home, and Gromyko on August 5th, 1963.