Thanks for looking Dan. Are your comments made from having looked in a paper copy of Sing Out Volume 33 No 1 or are you commenting from your knowledge of the magazine's format? Volume 33 No 1 does seem more likely because Sing Out as I recall was not a very thick magazine.
As to the song: it has a number of titles and many variations - with several Mudcat threads devoted to it
Hi Roger Rum
Roy Roger Um / Hi Ho Jerum
Hi Rosherum
DIGITRAD LISTING HERE
The firstof the above Mudcat threads says that Bob Cooney sang the song which is interesting because he was a political Commisar with the British Battalion in Spain. Perhaps he was the Brigader who contributed the song to Sing Out?