Seat Belt Extenders

[missallpc]

Any seat belt Gurus out there? I recently installed 3 point belts in my '57 3100 and when I put the seats back in the belts in the middle are too short to pull all the way through between the seat and the seat back. I'd like to find a click in extender instead of removing the belt and having a new piece sewn in. My problem is that there many different type buckle. Type A & B are all I could find online. I called the Mfg but they said they installed so many different buckles that there is no way to find out (not sure if that's true). I've attached a photo of the buckle tongue. The dimensions of the tongue are: hole 20 x 14mm and the outside 32 x 40. Help! Thanks in advance, Paul