Ask Phil's glossary says that O.T. stands for Obchodni Tiskvina, (Czech) overprint for commercial printed matter. The stamp is listed in Stanley Gibbons Simplified catalogue under Newspaper Stamps, SG N332(1934).
I know this is an old thread but I had a question about this stamp and wondering if anyone could enlighten me. The SG listing is imp.The one above appears imp. only horizontally, where as I have and imp. and a p 12 x 11.5. Are these unofficial perforations?