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I have never seen a seatbelt welded to a car frame. Replacement car seat lap belts are bolted through the chassis, with a big washer on the back side. The keychain carabiners in the photos are placeholders and listed as such. They were replaced with rated equipment.The safety concerns surrounding antique motoring are myriad. Was eventually involved in a t-bone accident in this car. These old cars simply crumple up like cans on impact.
Here are the two alternatives you have with an antique car that didn't come with any seat belts.: bolting in aftermarket seat belts or making a latch point for the car seat. The keychain carabiners pictured were placeholders so I could photograph the project. They were replaced with load rated carabiners for actual use. In the final analysis, antique cars are inherently dangerous, lacking even modest passenger safety consideration in their design. This addition enables these cars to transport car seat passengers with equal safety as the seat belted passengers. Which is to say, marginal at best. From the greater perspective of impact designed and tested autos.