Thanks, Farmington, for the input. So...we had it reversed all along...and...
I'm starting to piece together what think has been happening. I think our Welsummer, laid the first darker/speckled egg (see Post#22 in this thread), and within a day or two our Black Rock starting laying the noticeably lighter brown eggs. We wrongly attributed both eggs to our Welsummer who matured soner.
In hindsight, in the PHOTO above, the smaller, darker egg was from the Welsummer, and the larger, light eggs have been coming from the Black Rock.
Right as both hens started laying about 2 weeks back, the Black Rock must have suddenly started "aggressively pecking" our Welsummer...to where she is featherless on her back (I posted about this
here asking for help). I think this pecking stressed out the Welsummer to the point where she stopped laying (except for one, weird shell-less egg we found in the coop and couldn't explain at the time).
So yesterday, we applied Pine Tar to our Welsummer's wounds, and I think the desired effect is that maybe she's now being left enough alone to resume her interrupted egg-laying. Because, voila!, this morning a second, dark-speckled egg (like the one in Post #22). Coincidence?