Scott #R184-189 were issued in 1900, open-numeral surcharges on gray stamp depicting an allegorical commerce statue.
In 1902, a similar series was issued (R190-R194), but on green stamps and the surcharge was filled with an ornamental design.
Apparently along the way they had some production issues and used the wrong surcharge on the wrong stamp, resulting in the green stamp being overprinted with the 1900 surcharge. This is listed as Scott R191a. There also was a variant with the surcharge in violet, Scott R191b.
Additionally, there were doubled and tripled incorrect surcharges, Scott R191c and R191d.
I just received the item below, a single stamp and attached partial stamp, which I believe is a quadrupled surcharge, not listed in Scott.
The strip across the center is water soluble varnish, an anti-reuse measure. All examples of the incorrect surcharge I have seen pictures of have this strip.
There are two images below, the first being the stamp without annotation, and the second being the same image but with my indications as to the individual surcharges.
What do you think? Triple or quadruple surcharge?
For reference, here are the images of R191d shown in the Philatelic Foundation archive. These were certified as triple surcharges, but they are from an era when the revenue expertise was not as strong as it is today. I see evidence of a 4th surcharge in some of these images.