20th-century revenues used illegally as postage on covers are neat items, but as a general rule are not extremely scarce or valuable unless there is something extremely unusual about the cover or usage. Simple examples are usually $15-30 items.
Well, last week on eBay I stumbled across a usage I’d never seen before: revenues used as postage dues.
As Bart pointed out to me, it would not have been unusual for a post office to have revenue stamps on hand as there was a parcel post tax during this period. You occasionally see parcel post tags that have revenue stamps affixed. They are fairly scarce, but even though the revenue stamps are used right along side postage stamps, they are NOT illegal usages (despite the fact that some sellers try to promote them as such).
This letter, on the other hand, does not fit that usage. Even though the stamps are not tied to the cover, the cancel clearly matches the town of the addressee (see bottom pic that shows an enhanced version of the cancel).
Zion, Illinois is about 3 hours due north of me, right on the IL-WI border.