I am also curious about whether the barrel number matches the action. A rebarrel would account for the express sight. However, this particular express sight appears different than the ones usually found on BSA sporters--the standing leaf is thicker and there is no ladder. Could you please post some close-up photos? I'd guess that the backsight was fitted later.
The LEC designation, royal cipher and date make it clear that the action body (at least) was a military weapon and not a sporting carbine.