hyrax222, your byf 41 2917 no suffix falls within the serial number and suffix ranges posted on this site-the earliest being 5200n, and latest reported 276b. From the reported examples, E/135 with E/655 mixed inspection marks has been recorded at approximately sn. 4801. The double E/135 inspection receivers noted at about sn. 7478 (2917 moves that up) and continued to the end of production in 1942. As more examples are logged, the overlaps and ranges may be narrowed.