I can't find any reference to that block number. I've looked here: http://www.mortec.com/bbc.htm and Corvette books. The only casting numbers that I can find that come close to the one you provide for a 427ci is 3869942, 3855961, 3855962, 3869942. Are the numbers clearly legible on the block?
1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
427 casting #
I came up blank too. Can you take another look at the engine. Sometimes the numbers are poorly cast and sometimes one digit will just plain be wrong...a mistake. I would guess it to be the 3869942 casting unless it is an obscure truck number or something.