How would you fill in job application section asking for current and past salary??

I'm applying to a position that requires you to apply with a completed employment application form (an old school one that you have to download, print and fill in by hand) as well as a resume and cover letter. On the application form, they ask for your salary at your current and all previous positions.

How would you answer this? I don't really want to say what I'm making now because the position I'm applying for would be about a 15K pay increase so I don't want to give the idea that I would take less. I also have about 10 years of work experience and I honestly cannot even remember what I made at some of my previous jobs. I'm reluctant to leave it blank so that I don't look like someone that doesn't follow the rules. Ideas??

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