Hoover clip on plug patent expiration year

[2011hoover700]

Hi I'm wondering when hoover's clip on plug patent expired.
Thanks for shining some light on that subject.
2011hoover700

 

[dysonman1]

Without knowing when it was patented, but knowing patents expire in 18 years, the first Hoover I know of with the clip on plug was the 150 of 1936. So by 1954, the patent would have been expired. I've seen mid century vacuums with clip on plugs, so that would be about right.

 

[2011hoover700]

Speaking of clip on plugs did the kirby model 514 originally have 1?
I'll check into many more vacuums from 1954 and newer.
Thanks for clearing that 1 up.