Contact them?
It looks like they are still in business ..
http://www.clarkeus.com/
Please read the history
http://www.clarkeus.com/aboutus/ourhistory.aspx
You will see they started out making sanders, which is on your motor "Clark Sanding Machine Co"
Excerpts:
[COLOR=#231f20; font-family: Arial, sans-serif; font-size: medium; font-weight: normal; line-height: 22.4px]"Through the mid-20’s demand increased, and Clarke added a handle to his sander for use in wood floor restoration. He also added a line of vacuum machines to the existing sander products to make quick work of jobsite clean-up"[/COLOR]
<p style="font-family: Arial, sans-serif; color: #231f20; line-height: 1.4; font-size: medium; font-weight: normal;">
Clarke Milestones:
</p>
<p dir="ltr" style="font-family: Arial, sans-serif; color: #231f20; line-height: 1.4; font-size: medium; font-weight: normal;">1956 – PS26/PS30 Electric, Gas and Propane Floor Scrubbers and DU-8 Rental Sander
1958 – First battery powered floor scrubber, the Clarke-A-Matic BP 20
1959 – Clarke introduces the FM Floor Maintainer
1963 – TB Series Automatic Scrubbers
1965 – First Dual Motor Upright Vacuum Cleaner, the Model 550
1969 – First wide-area vacuum, the 580 SpaceVac </p>
<p dir="ltr" style="font-family: Arial, sans-serif; color: #231f20; line-height: 1.4; font-size: medium; font-weight: normal;"> </p>
<p dir="ltr" style="font-family: Arial, sans-serif; color: #231f20; line-height: 1.4; font-size: medium; font-weight: normal;">Also found this from 1927 ..</p>
<p dir="ltr">[COLOR=#231f20; font-family: Arial, sans-serif; font-size: medium]<span style="font-weight: normal; line-height: 22.4px;">"CLEANER Different in price, different in weight, different in construction as well as convenient, easy to handle, made to operate in one hand without effort — these are a few of the claims made for the Clarke Vac-All Electric Cleaner."[/COLOR]</span></p>