I don't know what the current "laws" are (I really don't), but I do remember when seat belts became common law, there was an exemption, that if the vehicle was a farm truck or if the vehicle was originally manufactured with no seat belts, they were not required. Time may have changed, but I do know, people who drive antique cars, don't have seat belts installed. And the farm trucks I've driven (in Indiana) are not required to have seat belts (in the older models, which I've driven .. just recently.)
It wasn't THAT long ago, that if your vehicle was licensed as a "truck" seat belts were not required either. That did change, so all new "trucks" had to be installed with a seat belt. The "truck" rule was a way that a lot of folks got away with not using seat belts, by purchasing "trucks" or finding a way to have their vehicles titled as a "truck." Indiana finally caught up with this and finally made it mandentory or all vehicles. Still... I don't know about the "Grandfather" rule. You might check with your local DMV or BMV.
Edit! I did find this on the Internet from the Indiana codes:
IC 9-19-10-5
Retail sales, leases, trades, and transfers
Sec. 5. A person may not buy, sell, lease, trade, or transfer from or to Indiana residents at retail an automobile that is manufactured or assembled, commencing with the 1964 models, unless the automobile is equipped with safety belts installed for use in the front seat.
As added by P.L.2-1991, SEC.7.