John,
I purchased my HD wagon springs from FCP. I know that IPD has some different offerings, but don't know the specifics about wire diameter, or spring rate. I know the cost was significantly more than the FCP ($90 for a pair I believe). As I posted, those springs are made in Sweden, so perhaps Lesjofors but I don't know that for a fact. The spring rate I posted for those wagon springs, calculated vs. Anthony Hyde's posting. I neglected to deduct the paint thickness, so my calculated rate should be a bit lower (I need to edit that post).
Duane