Most new springs such as Kings Springs SL & SSL are usually variable rate (ie coils clooser together on one end) so you get a stiffer, lowered spring that won't fall out....
Are you sure it is not slotted? Sometimes the retainer clip can be deceptive, should be a half height nut on the flexi hose end clamping a spring clip onto the slotted bracket...
Cheers
James