Searching through the online archives of some US and international law journals, I found "The League of Nations Official Journal" with some interesting material concerning the International Military Commission set up to monitor and verify the withdrawal of non-Spanish combatants from Spain in 1938. The ICM was set up by the League of Nations on 30 September 1938 at the request of the Spanish government and consisted of three commissioners. One of these, General Jalander of Finland, was appointed president of the commission. In addition, the ICM had a secretary and nine attached officers from various countries. The ICM constituted itself in Perpignan, France, on 14 October and left for Barcelona two days later. On the 31st October 1938 the Government of the Republic of Spain presented the ICM with a Plan of Withdrawal drawn up by the Ministry of National Defence. The work of the ICM continued into the first part of 1939 [this summary of the ICM’s history is based on League of Nations Official Journal, Vol. 20, 1939, pp. 125-6].
The League of Nations Official Journal for February 1939 contains a translation from Spanish into English of the Spanish government’s Plan of Withdrawal. Section 5 of this document is of interest to vexillologists as it sets out the means by which the ICM and their members were to be identified. After describing the identification badge issued by the Spanish government to ICM members, we find the following provision: "The cars placed at the service of the Commission will carry a flag bearing on the one side the colours of the Spanish Republic and the initials of the Commission, and on the other a white ground with the initials "S. de N." [League of Nations Official Journal, vol 20, 1939, p. 139].
Given that the reverse of the flag were to carry the initials of the League of Nations in the Spanish language - Sociedad de Naciones - we can assume that the initials on the obverse were also in Spanish - "Comisión Militar Internacional". In other words the flag carried the initials "C.M.I." on the obverse and "S. de N. " on the reverse. The Spanish government’s plan did specify the colours of the initials. It would seem reasonable, however, to assume that a colour standing out in contrast against yellow and white was used. I have assumed the lettering to have been in black. All in all this would produce a flag with a front and back like reconstructed in the above two images.