The factory rear suspension on a '49 3100 is leaf springs. Any coil springs you saw are not original.
But you are comparing lever style shocks to coil springs.
Are you sure you are looking at the right things? The earliest AD trucks had lever action shocks with a migration to tube shocks happening about 1949. I'm a little unsure on that date, but could it be tube shocks you are looking at and not coil springs?
