Looks like the date code makes it a Jan 1942
You are correct….”BL” is January 1942. Not completely out of the realm of possibility but certainly not the norm. Often the barrel dates predate the weapon as explained earlier but generally not by an entire year, although I did read on one of the forums sometime ago of the same coding on a 474xxx Model 11 but the individual didn’t say whether the SNs matched and he didn’t provide any photographs. I personally have not seen a “BL” example that was coded on a 1943 serial number but certainly not to say they couldn't exist, it is possible since it predates the weapon serial number, just not the norm. Barrels were interchangeably swapped but if that were the case, the serial numbers would likely not match the receiver.
According to the archived factory ledger, #456516 was the last Model 11 produced in Jan 1942, so there are a few possibilities. One, not the original barrel (matched to Model 11, #473620) and was changed out at some point; two, barrel was sitting in a bin for a year before being installed, or three, improperly marked when originally coded.
An interesting note is that the folks at Remington provide production dates that often don’t jive with any documentation or author. As an example they said mine was produced in 1939, when in fact it was clearly produced in 1943, without a doubt. They have admittedly said that record keeping back then was not so good, so depending on the year most take that with a grain of salt.