8 is the year (1958, 1968, 1978, etc...), BL is unknown (I've never seen one with those letters), and 254 is the 254th day of the year (Tuesday, September 10th, 1968). Of course, this is assuming the box was built in 1968. If it was built in 1978, then it would be Monday, September 11th, 1978.
Ed