And now I've learned a new word, "setoff," which I previously knew as only as "offset". Linns philatelic glossary says,
Offset:...2) The transfer of part of a stamp design or an overprint from one sheet to the back of another, before the ink has dried (also called set off). Such impressions are in reverse (see Mirror image). They are different from stamps printed on both sides.
Don't measure, but compare to a template made from a known flat plate in the same series, e.g., another 484 you've already identified. Measurements vary and Scott listed measurements are not definitive.
It looks to me that because of the way the perfs cut into the frame that there was a large margin on the right side and for some reason, the right perfs were trimmed off. Also from what I can see from the scan, it appears that the stamp is a 493, type I.
