Replace those old corroded bolts with heavy duty zinc plated new bolts.
These are very high quality bolts. Same 8mm thread and small 10mm head as the originals. 8 is enough for two front seats.
W114 W115 W116 W123 Front Seat Attachment Bolt Set (8)
$6.80
SKU:
INT-SEATBOLTS8 
Product Details
This is a sample of the nut plate they go into on the back attach point on the W123 front seats.