Mike you can find info here
http://www.littelfuse.com/cgi-bin/r.cgi/en/prod_series.html?SeriesID=129&LFSESSION=pkqP0Dgxlf Older fuse holders in American homes could accept a fuse from 15 to 30 amps.
Many times the homeowners would replace the 15 and 20s with 30s.
So now we screw in a permanent adapter that allows only the correct size fuse replacement.
