Just wondering if anyone has any information regarding Retina serial numbers beginning with BK instead of the North American import EK series. I recently purchased a IIa with serial number BK833306. The B appears to be a modified K with an additional line engraved to form a somewhat awkward B. The only reference I could find was a similar request for info from a gentleman with a IIa with serial number BK833239. Was this used for British imports or was this an attempt to bypass import duties?