Phil-
SG's British Empire makes no reference to a color variety for the 1912 series 50c (Scott 209). There is an inverted watermark that Scott Classic doesn't reference, with a significant premium.
Here is my Scott 209. (SG 314, for anyone that can doublecheck my catalogue work.)

The 1921 series 50c also is a one-color-variety affair, albeit both dies. That one would be a mult script CA watermark, versus a mult crown CA for the 1912. Both are chalk paper, so no help there.
Any chance one faded in soaking, being chalky paper? All I can think of.
Keeping it around might only confuse things...best to send it to me.

Collin
[edited to add one more hyphen.]