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Speaking of Electrolux dusting brushes with black rubber bumpers... the ones with black bumpers (early XXX) and gray bumpers (later XXX) have been far easier to find over the years than the ones with the grayish-blue bumpers, which came out with the LX.
The rubber material these later ones were made of was inferior and it cracked and deteriorated over time, gradually crumbling away. I've also seen replacement XXX handles made of the same grayish-blue rubber and they, too, are a mess. It may be that the new grayish-blue dye color was not compatible with rubber. There are some products, such as oil-based materials, that degrade rubber. It's possible that the new grayish-blue dye contained oil and they didn't know at the time that it would adversely affect the rubber.
Also, the same thing happened to the gray rubber handles used on the Kirby Model 508 (and, possibly, early runs of the 509). It had a ribbed rubber handle in the same style and design as the 505 but was gray instead of black. I have several black 505 handles in perfect condition but have never seen a gray one that wasn't all cracked and deteriorated. When the 509 came out the handle was changed from rubber to gray vinyl, and those have held up very well over the years as long as they weren't abused.