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.
Related Products:
PDF
$9.95
Front Seat Repair Manual by Kent Bergsma - Download
Fits all chassis: 107, 114, 115, 116, 123, 126, 201 and 124
Is your drivers seat starting to sag more than it use to? Are you finding it is just not comfortable any more on long trips? Maybe it tips to one side and you are constantly trying to compensate your weight to the other side.
$97.50
Lower Front Seat Spring Sag Repair and Restoration Kit
For ONE 116, 123 and 126 Chassis Sedans, Coupes or Wagon Seat Bottom
For ONE 116, 123 and 126 Chassis Sedans, Coupes or Wagon Seat Bottom. This unique kit is designed to help you repair and restore one broken down front seat bottom so common in the 116, 123 and 126 chassis Mercedes built from 1973 to 1991.