Nakahara et al write in Identification of Vitamin L1 (Anthranilic Acid):
In 1934, we demonstrated for the first time the existance of a specific dietary factor of lactation, since designated vitamin L1, and the progress made in our investigation on the subject has been communicated to the Imperial Academy from time to time.
Thus in 1936 we reported that there exists in addition to vitamin L1 a second factor necessary for lactation (vitamin L2), and later showed that yeast nucleic acid has the action of this second factor.
Is the claim that L1 and L2 are necessary for lactation and thus vitamins true? Wikipedia says there's evidence that it isn't. Wikipedia cites an article by Michael W. Davidson of the Florida State University that itself itself lacks sources.