Does anyone know the difference in spring codes?

I believe I have U for the rear and N for the front. I'm considering buying some stiffer springs from a salvage yard, so I want to know what's what.

Thanks.

 

I had to go through this once while replacing the spings on my Sport. Seems to me that the guys at the bone yard had access to this information. I may have gotten it from the dealer though. Wish I had a more definitive answer.

 

Thank you.