Wonder if the postoffice would have accepted these sheetlet unmarked labels as valid postage, or possibly there were other valid stamps placed on the envelope along with this label--I only have the remnant pictured.
Can only be speculation. The CDS (circular Date Stamp) part of the slogan, would have been on the left.
From my own experience, I have used, and received stamp "tabs" on mail,
if I am posting and there is a gummed tab, on it goes.
The tab may have gone through the machine canceller solo, but I doubt it.
Most people are honest, sometimes a collector may and try to post a tab as a bit of hijinx. If that were the case, I think the entire cover would have been salvaged.