Crocker1 reported a case of erythema multiforme in which the mouth alone was affected, and which "showed attacks of bullous aphthae beginning on the buccal mucous membrane, and spreading over the tongue and mouth without any skin lesion. The affection occurred every two or three months. After being under observation for over a year, erythema iris lesions appeared on the back of the hands in one attack, and the patient then remembered that he had had a similar attack some years before."
So far as I can determine from a review of the literature and current textbooks, there are no other reports of cases of erythema multiforme confined to the mucous membranes.
Osler2 reported eleven cases of erythema multiforme with visceral complications. In two of his cases in which the attacks were entirely visceral there were no skin manifestations. In none of them were the mucous membranes involved
JOHN BUTLER. ERYTHEMA MULTIFORME CONFINED TO THE MUCOUS MEMBRANESWITH REPORT OF CASE. Arch Derm Syphilol. 1922;6(1):1–5. doi:10.1001/archderm.1922.02360010004001