It's hard to tell the difference between 39 and 40 Mercs without the headlights. One way is the wipers on the bottom of the windshield on the 40(windshield doesn't crank out). Another way is by the tail lights. On 39s they looked like toilet paper cores sticking out sideways
Snyder's in Ohio is mostly a Model A and Model T parts dealer but they also do antique seat springs as per their ad in Antique Automobile. You could try them, just look up snydersantique.com
That seam that runs parallel to the outside edge could make it Plymouth or DeSoto around 30. There were other cars with that seam but not Ford, Chevy or Packard