73 Duster rear seat belt anchor bolts - what are they ?

Hey gang,

I am putting the interior back in my 73 Duster and I see I am missing the nut that holds the passenger side rear seat belt retractor assemblt in place. It appears to be a fine thread, but I've tried every size nut I can find / think of and they are either too big or too small. Anyone know what size nut I need ?

Thanks a lot !

Trevor