Well, I had two more rubber eggs this morning under the roost- these hit the ground and burst open, so I had to shoo everyone away this morning as they wanted to eat the yolks, which were laying in piles of poop. Yuck.
Based on where these were laying, it's clear they came from one of my Golden Laced Wyandotte pullets. They were laying almost on top of each other, so I am thinking they were from the same pullet, but can only narrow it down to two ladies who were sitting tightly bunched on the roost.
It has been almost 3 weeks since I wrote the first post, so I guess now I'll start to see if there is some sort of pattern to this- I did not have any strange eggs for three weeks, we'll see if it gets on some sort of cycle.
They have plenty of free choice oyster shell and are still on the same layer pellet, so not sure there's anything to change at this point. Will just continue watching.