What does the number on the back of my Polish Jump Wings mean?

I was wondering if anyone knew what the number on the polish jump wings indicated. I was awarded these wing with the number 15 on it but the back had a thick bolt like pin and in order to place it on my uniform I had to buy another set at clothing and sells, which I could only find the number 7... what does the number mean? The polish unit? And where can I find this answer?