Some years back Dave Dahle explained to me, and now I can add a picture.

A copper disconnect blade ( shown on the left) is interchangeable with the normal fuse tube for a given style of cutout.

When a cutout is fitted with a "disconnect blade" the cutout serves as a line switch instead of a fuse.

Douglas, another Icon member adds "Otherwise known as a "slugged" cut-out. Oftentimes we used them on voltage regulator and recloser bypass set-ups for maintenance. "

CLM Industries, Scarborough Ontario Canada