According to the 1991 service manual (pages 20-38, 20-39), the center and lower seat belt anchor bolts (attach to the seat) are 7/16 -20 UNF. The service manual does not specify the length or whether there is a shoulder. You will have to confirm that once you have extracted the bolt. The service manual also does not specify the grade of the bolt. You will have to confirm that by checking the markings on the bolt head (check the head on an undamaged anchor point mounting bolt).
Most (all?) of the major bolts that bolt into the NSX's aluminum structure have a dacrotized coating to reduce the risk of galvanic corrosion between the steel bolt and the aluminum body structure. Major bolts on the outside of the car are all dacrotized. The service manual specifies whether the bolt is dacrotized by marking it with a star in the assembly pictures. In this case, even though the anchor bolts do not bolt to the aluminum body structure the service manual specifies that the anchor bolts are dacrotized. Not sure why.
If you want to replace with a dacrotized bolt you are definitely not going to find that at your local hardware store. You can try a fastener specialist vendor; but, you may have to order on-line because dacrotized bolts are not super common. In a pinch, zinc coated bolts can be used as a substitute for dacrotized bolts since zinc and aluminum are relatively close on the galvanic series chart. Definitely do not use any kind of stainless steel bolt as a substitute for a dacrotized bolt as stainless steel is 'off the chart' compared to aluminum. Your hardware store will also probably not have the correct grade of bolt as I suspect the anchor bolts may be high strength bolts.
Go figure that Honda would mix in some non metric bolts on a car where everything else is resolutely metric????? I would be careful when test threading the replacement anchor bolts into the seat because that non metric bolt just seems so weird.
