If you go to the vet you might want to copy some pictures of squamous cell carcinoma in poultry. There are a few online which I shared here. That way the vet might become familiar with the disease. Many vets know some about chickens, but not about the types of tumors or serious illnesses. My...
It wouldn’t be Mareks if it turns out to be squamous cell because Mareks causes lymphoma tumors. But I really hope that it is fowl pox. Post pictures if more lesions or scabs show up.
That doesn’t look like fowl pox to me. Squamous cell carcinoma or keratoacanthoma can cause crater-like lesions on the body. I would observe her for more of these lesions. Hopefully, it is not that, but looks similar.
https://sesc.cat/en/squamous-cell-carcinoma-in-laying-hens/