Seat belt bolts are, for the most part, size 7/16-20 the world over - even on non-USA cars.  GM switched to M12 a number of years ago, now they have a global standard and Europe insisted it on being 7/16-20, so new GM designs will (again) be 7/16-20!

The US MVSS  209(motor Vechicle Safety Spec) originally specified 1/2-20 and 7/16-20 (some early cars used 1/2-20), it now has added "or metric equivalent".  Funny thing is, US made cars had seat belt bolt standards to pass, imported cars only had to have the entire system pass, so some imports had M10 size bolts, but even cars that were never going to be made here generally used the 7/16-20 size.

So what Dave says below is true - the bolts are actually 7/16-20.

John -
Who has has been involved in testing and design of "few" seat belt bolts for a few decades now.

>Just so happens that 7/16-20 UNF is practically identical to
>M11x1.25; 1% on diameter and 2% on pitch.  For a short thread
>engagement this doesn't matter.  The UNF and ISO thread forms are
>slightly different, but not enough to matter here.

>Yours,
>David