I believe Dave has it right. When Seat Belts were an option, which I believe were available in '61, that must be how the factory (or dealer) did it. Although they may have used Plastic "Moldings" to cover the slit.
Edit: Just for a little extra info, I just saw where '62 was the first year for Factory Belts. Before that they were Dealer Options.
-Parting Out over 75 '58 to '73 Full Size Chevy Cars-
-National Impala Association
-Vintage Chevrolet Club of America
Last edited by 62BillT; 04-17-2016 at 09:15 PM.