ProfessorLinsay has called my attention to the need for an examination of these compounds, with a view to ascertaining whether they are ever used in the Singular by Latin writers, or always in the Plural.
From a scholium, probably by Donatus, on Aen. X. 173:
(Ilua) insula inexhaustis Chalybum generosa metallis, has come a gloss which appears in the Ansileubus Glossary (just published in Vol. I. of Glossaria Latina, Paris, Les Belles Lettres, 1926) in two forms:
FR 205 Frofra (!): insula Tirreno mari in quo (qua) ferrifodina <e> exercentur, RU 77 Rufa: insula Tirreni maris in qua ferrifodina exercetur.