Took the hint and looked at the rest of the 1928 Offset Printing, #'s RD27a is an inverted overprint and RD28 has an un-lettered variety, a double impression of the stamp. But am still unable to see anything except the dropped S in the overprint of RD25.
I thought about the spacing between the lines, but had no way to measuse the image, or any visual reference as I don't collect Revenues. I do have a small collection of the 1875 Special Printings of Departmental Stamps. There were several errors in the overprintings of those stamps with the word SPECIMEN, which are listed as minor numbers and I do actively seek out those varieties, just as I do with the U.S. Banknote varieties. The SPECIMEN overprint has a dropped S like I noted on your stamp, also not listed.
There were billions of these stamps issued, minor differences in spacings are to be expected and I seriously doubt that any are uncommon. Also, that is not a "dropped S" All the letters of "stock" are even at the bottom, it is simply marginally thinner than some other S letters on other stamps. This series was used from 1917 to 1940 and there were many millions of transactions needing revenue stamps every year. Anyone with a decent quantity of any of the low values and a 10X glass will find very minor differences if they look for them.
No listing unless the difference is at least one full letter height, perhaps 2mm. With all the plates needed for the lettering over the decades minor differences are sure to occur, so unless it's a major difference it has no importance in my opinion.
The Scott Specialized catalog is certainly not the end-all list. Minor varieties like this show the value of the specialty societies through their journals and detailed catalogs. This is the type of variety I would expect to see written-up in the American Revenuer with much more research on othe measurement differences on this and the other denominations - a thorough study of the entire issue, rather than one lone stamp. Then it might make it into Scott.