Windsor strikes - I would believe they are in the @.6 range of .999 silver content , because they were made by GDC Company & in what appears to be the Standard $10.00 format...
The only strikes I'm familiar with having noticeably smaller center "silver insert" sizes are the ones issued by RWM company.
With the RWM $10.00s you can see more brass at the pressed/silver boundary line.
The RWM 12oz. Strikes were made from very poor Quality Silver flat rolled material in my opinion - full of pinhole porosity lack luster polish/ finish, etc. -- but still nice to have.
It may be possible that some "S & G" mintage's are "thinner thickness" silver in the later years after 2004, but not as common as some bullion dealers claim...
I've had dealers wine about some .5 centers and not do a deal unless you acquiesces to their demands/offers and I know that not one strike in my sale to them was a .5---- they were all .6 older LM , CC, GDC Mintage's .

A couple years ago times were tough , taxes were due and silver was up. I didn't argue over the .1 under valued spread they offered, it wasn't worth it.

Small Rant -- hope it helps

Gabe