Which 501 are you getting? I have three. Two are later models with the light blue color scheme and rubberized bags, while one is an early model with gray plastics and a red and gray twill bag. I've never laundered any of them. I've always just used my shop vac to vacuum them out. Seems kind of redundant to be vacuuming a vacuum, but it's the best way I've found. That said, all three of them are now fitted with paper disposable bags. They're getting harder to find, and thus increasingly expensive, but I see it as a worthwhile investment.
I've also got a plastic Dirt Devil model 08100 hand vac that obviously shares some DNA with the Royal prince 501, but besides being a little bit lighter weight, it'is not nearly as good a machine. It uses paper bags as well, which are quite readily available, but those won't fit the metal 501s and I don't see a good way to modify one.