Scott #1625B (Tag Omitted) Value?

Here are three more photos (2 under UV and 1 under natural light). I think these are clearer than my previous images.

The middle picture left is telling me that your stamp is indeed a stamp which has the taggant removed by some method.
1. There is a residual tagging vertical line at right, above the "c" of 13c.
2. At right of the top cancel line, there is residual glow of the taggant. Thre is also residual taggant between the first and second cancel line.
3. Just above the "United States" there is residual glow of the taggant.
4. Along the left edge of the stamp, there are splotches of residual taggant that still glows slightly.

Points 1-4 form the outline of a tagging block that was shifted to the left by about 1mm. Point 2 shows the problem I mentioned in a previous reply. The areas around and below the cancel lines are the most difficult to "clean of taggant".

All in all, I am now more than convinced your stamp had the taggant removed intentionally or unintenionally.

The "normal" stamp at right has some curious features as well. It seems there is a untagged ghost at left of the building identical to the building. It has also lost a lot of the taggant as there are areas very weakly tagged.

Thanks for sending the additional images! I agree tagging is present on the left stamp as drkohler described but I think it could be from a weak application.

Along the same vein I also found what I think may be 1622f. I wonder what everyone thinks...I don't see the same problems. I also think this is a much less uncommon stamp. The one I think is "tag omitted is on the left:

Yes I don't see tagging either.

my, my, talk about overshooting! If you with the question stamp would put together a small sampling of 1976 stamps (plenty of commemoratives then) especially to include more of the 13c Flag (sheet or coil), you will get a good look at the unevenness inherit in genuine applied tagging to these stamps. (yes, even among1976 stamps there are some differences in applications, the gravure large size commemoratives will usually differ from the steel engraves small size regulars, but these were all overcoated so good to see the variety of results you see under UV by the time they got to you in used conditions.
yes so the press mat or cylinder applying the liquid tagging is not going to paint an even coating consistently.
Now you add a light application (very common on the printed by the hundreds millions to billion regulars), and the more common cause than "soaking" (which rarely affects tagging if normal water soak) -- over time, simple exposure to sunlight or indoor light (especially fluorescent) can fade it, just as some more fugitive inks used once in awhile. Exposure can happen to the mint stamps keep in mind, as well as the stamp on the letter arrived and sits around on someone's desk, before the stamp is eventually clipped and added to a mix then sold and so on.
You mention expertizing, certainly not worth it for used ones like you've seen with any evidence of a trace remaining.