Thanks Rhys. Which case has the BN? Left case or right case?
Very unusual to have any letters in a 1941 BN. I have no letters in photos of any authentic-looking BNs for 1941. Here are two 41 Knuckles for example.
But the factory did use the letter E in Knuckle BNs for 1940.
Could you post a photo of your BN please. And a photo of your SN. Thanks.