Tagging Omitted Vs Untagged

I know there has to be a way to differentiate between a stamp that Scott lists as "Tagging Omitted" and "Untagged". I reference the listing for 2169a & 2169b. If neither glows under the UV light, then there has to be another way to distinguish between the two. It would be the same for other stamps like 2173a & 2173b for example.

Do you have a Scott Specialized? There are notes under the two that you have mentioned that address this.

Sometimes one can distinguish between "untagged" and "tagging omitted" stamps ... and sometimes one cannot. In the latter case, one must assume that the stamp is merely "untagged" (presumably the most common variety). As eyeonwall has suggested, consult a Scott U.S. Specialized Catalogue for the details. But .... let me share a story with you.

About 15 years ago, I found myself looking for a specific "tagging omitted" stamp in the Great Americans series. Scott 2170 (Paul Dudley White) was first issued tagged ... and later untagged as 2170a. Moreover 2170 was issued only with dull gum while 2170a can have either shiny or dull gum. Hence a shiny gum White can never be "tagging omitted". The Scott notes indicate that, to verify a dull gum stamp as 2170b ("tagging omitted"), the stamp must have a plate block number (plates 1 thru 3 are 2170 while plate 4 is 2170a) or a tagged stamp attached to it. During my search, I happened to mention this difficulty in identifying 2170b to a stamp dealer who specialized in modern U.S. definitives. Much to my surprise, he indicated that Scott didn't have the complete story on this stamp. He told me that 2170 (tagged) and 2170a (deliberately untagged) were printed on two subtly different paper types that, for mint copies, could be distinguished via the dull gum striations on the back of the stamp. I no longer recall the details other than for one the striations ran at a distinct angle to the vertical while for the other the striations were vertical. When I eventually obtained a 2170b ("tagging omitted") ... verified since it was a plate block ... I compared the striations on my 2170 with the 2170b block as well as a 2170a plate block. Not an easy task ... I had to hold the stamps at an angle to a bright light source and stare at them until I could distinguish the striations. Difficult then, probably impossible years later with aging eyes & cataracts. But I do recall that my stamps seemed to agree with the dealer's claim. Since then I've often wondered how accurate his claim actually was ... my brief fling with staring at the backs of 2170s proves nothing ... and whether similar details exist for other stamps in the series.

There was a publication by Steve Esrati that went into all the paper types for the different Great Americans series.

"There was a publication by Steve Esrati that went into all the paper types for the different Great Americans series."
This was serialized some years ago in The United States Specialist, monthly journal of the United States Stamp Society (usstamps.org).