According to this site http://www.rafweb.org/OTU_2.htm the codes DK, OP and RD were used at 32 OTU between September 1941 and October 1942. According to the Beaufort File both OP and RD were used on Beauforts using also the same identification letter, thus N1005 is shown as OP.A and also RD.A. This is somehow rather confusing. I am sure that there must have been a cut off date from one code to another. Can someone please explain this.