My apologies to Febi-Bilstein!
I replaced my struts last week (98 S70 NA 218K) and I purchased a new Boge (Volvo) spring seat to install on one side because the top strut nut was spinning for about the last 8K miles and I assumed the Febi spring seat I previously installed had failed.
I discovered that the spring seat is just fine and actually looked to be in very good condition. The reason the top nut was spinning is that the cross shaped nut that goes on top of the spring seat had become loose.
The Febi-Bilstein spring seat is made in Germany, looks to be solidly contructed and (contrary to what I had thought) is still going strong at 58K miles.