I have previously come across 5c stamps on the 1E4 paper. The two blocks in booklet format are on this paper. I have just realized that it is possible that the booklets were only printed on the 1E4 paper, and that this paper was only used for this purpose. If this statement is correct, then loose 5c1E4t are from the booklet printing, and this printing must have taken place on or soon after 1940, when the 1E4 paper was briefly in use.
Additional note (March 2009): Upon reading the article by Tenorio a second time, he specifically mentions that what I propose here is correct. The 1E4 paper WAS used for this printing, and all 5c1E4 stamps come from these booklets.