BE rears do not use a letter date. If it was a March build it would be stamped "0318", followed by the shift it was built on, 1, 2, or 3, and a letter "G" for the Flint plant.
So for March 18, 1969, 2nd shift, it would have to be stamped "BE 03182 G".
The BE was a '69 only code. Post a pic!