I'm not sure if he deliberately used them for postage to flout the rules or if he didn't realize that they couldn't be used as postage, since they looked like postage stamps of the period, overprint notwithstanding. All of the usages I have seen are right at the initiation of the tax, so information may have been sparse. Dunno.
I've seen several 2nd-day of tax covers from Ludwig Schill, each being all-over advertising covers. I own two of them, shown here:


Well, then last week the item below showed up from our favorite New Jersey seller, idiotically described as a "scarce cover with Interpol revenue cancel". Wait, what?
1. It's an overprint, not a cancel.
2. Interpol? In 1898... seriously? Interpol didn't exist until well into the 20th century.
I know that some of our favorite ethically-challenged dealers tend to get "creative" in their descriptions, but now they've apparently taken it to 11 and just resorted to "making up random sh*t that sounds good".
What makes it interesting to me is not just that it is a Ludwig Schill illegal usage cover, but the date. This is the earliest use of R155 that I have seen, illegal or otherwise. The first day of the tax was July 1, 1898 and this predates that by 2 full days.
Unlike June-dated revenue cancels on documents (I have a June 30 cancel on a battleship revenue and have seen another June 29 example somewhere), which could possibly be backdated, i.e., the stamps weren't actually affixed on the cancel date, it is very unlikely that this stamp wasn't actually used on June 29, 1898.

