The thing no one can explain is my double letter prefix serial number DB 55xxx
From this chart, 1927 was B56536
What if my bike were 1927 , but as it was made in Dublin, a D was added in front of the number.
1925-Second World War
Between 1925 and the cessation of cycle production during the Second World War, letters prefixed or suffixed the running frame number although no indication is given of the starting point of the numbers. They seem to reflect only the number of machines produced whilst each letter was in use. All dating would therefore be approximate. (Info from the Nottinghamshire Archives)
1925: A1
1925: B34181
1926/27: B56536
1929: E15693
1930: G94785
1933: L84682