Now you've got me wondering about what the heavy duty spring should feel like. Oops.
I think the only solution here is for you to send me a whole new Precise P18 with a heavy-duty spring pre-installed for extended testing in both hands, switching hands, over the course of three/four years. *nod nod nod* -Magnus
If David is not able to do that, you can just send me your P18 so that I can test it for your for a few months
Seriously, the HD spring is noticeably stiffer than the light spring. Just stand up your P18 on the table with a battery and atty installed. If the button pushes up from the weight of the unit, it's the light spring. If it doesn't, it's the HD spring.