Mine are 25mm Al 1.15 lbs each
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Not sure where the difference in weight is between yours and mine. My scale wouldn’t weigh 1 by itself so I stacked all 4 and it read 8.6lbs for all 4. Either way, ~2.1-2.2lbs each.
Yours are hub centric and 25mm thick, but not wheel centric as well.
Mine are hub centric at 21mm thick, and also wheel centric (same center bore as the wheel).
I’m sure the extra material to make the wheel centric portion isn’t the same weight as the extra 4mm your spacers have (I’m assuming my extra material is less than yours)
Both are 6061 T6 aluminum. Maybe it’s the stud material???