Does anyone know the size/thread/diameter/length of the two screws that affix the bottom of the seatback to the seat frame? TIA

 

i do not but your local Lexus parts department should have the answer.
have you called them?

 

Reviewing the service history on my SC shows that the PO had the car in the dealer shop 4 times for the seat back falling off. (02 with 96k). I removed the seats this week to have them recovered and neither had the two screws holding the back of the seats. When I looked closer, it appears that the flanges for the screws are stripped out. I’ll be using a bit large screw when I reassemble the seats.

 

I found the panel screws in a variety pack from O Reilly's. Cheap and with multiple sizes to choose from, it gave me exactly what I needed.

Paul