If it doesn't have a place for the oil filter it is almost certainly a 1955. In 1969 was the introduction of 4-bolt mains. beginning in 1969 many small block engines did not have the number on the machined plate, as an anti theft measure it was moved to the rough cast area just above the oil filter. You are correct on the year models it could have come from. the reason I put the above is to let you know about what range it could have been. You can find out a world of info. in the "Chevy V-8 Engine Casting Numbers 1955-1993" book published by Amos Press Inc. It is compiled by the staff of "Cars&Parts Mag." I hope this will get you going in the right direction.
