I've seen the "56" mark on other barrels, and it appears to be a proof mark, not a date stamp. I have another rifle with that stamp and a standard date stamp on it. Also, in the Knowledge Librarythere is another #4 Mk I LB and in photos 130, 131, 132 and 137 the same stamping can be seen, with the normal date stamp adjacent to it. This link might work (or might not)......
1941 No.4 Mk1 Long Branch Rifle
Ed