If it's 383 it has to be an after market crank, becauce the main jurnals are different size on a 350 blk and a 400 blk.(2.450/350---2.650/400 mains.)
But it's still hard to tell because some after market cranks are internally balanced. If it's an after market crank with ext. balance it will have as before mentioned a counter weight in the balancer and a counter weight on the fly wheel like a 400 blk/motor.
Zip.