US Scott #76 (Brown) Vs. 76A (Black Brown)

Outside my area of expertise here... Scott includes in #76 various shades of brown including "dark brown" but differentiates "black brown" as a separate number, #76A.

Looking through high-resolution images at Siegel's power search, while there are a few #76A that clearly are a distinctive shade, there are many other #76A (including expertized examples) that to my eye are no darker than #76 examples also on their site.

In my opinion the stamps shown below are #76 "dark brown", but the images at Siegel make me question whether they might be #76A.

Are there any litmus tests for differentiating between the two?

Unfortunately, you can't get an exact representation of the color using scans. I have made scans and compared the scan to the original and they looked different. I would say that your pair is certainly in the dark brown area and could be black brown, but the final determination can only be made some one who has seen the pair live.

Can't get to my U.S. Specialized right now, but I think the catalog number for the black brown shade is 76a, not 76A. In other words, that shade is a variety of 76, not a major listing of its own. Apologies if I am wrong on that. And I totally agree with Mr. Crowe that the pair would have to be seen live, not in printout or on computer monitor, to determine the shade.

You are correct, it is 76a for black brown and the other shades (pale brown and dark brown) being simply listings under 76 (brown).

Unfortunately, the 76a black brown variety is controversial among advanced collectors and expertizers. I have a PF certified reference copy which could be a very dark brown Scott 76. It is said that the true black brown variety is a very dark color like burnt toast.

that is a gorgeous stamp