I've got some genuine Kirby flexes that I recently obtained for exactly this. They have a kind of fluting on them and are very flexible and rubbery as you describe. See my Instagram post on the subject >>here<<
. The ones I got were a lighter grey, so while not original for a 50s Hoover, they are very in keeping with what you are trying to do if you chop the moulded plug off and pop a nice vintage MK plug on there - it works.
These are the ones I have:
But the photo doesn't show you the texture in enough detail, but if you've seen a Kirby you'll know what I mean.
I need £15 each plus £3 P&P if of any use. I could spare 1-3 but no more.