Not all bands have the date code. They are located on the outside of the bands about 2 inches from the clamping screw. The letters are about an 1/8" high and can be hard to see with a few coats of paint over them. The bands I'm looking for should have two dimples at each end near the clamp screw.
J = 1929
K = 1930
L = 1931
The number is the month starting 1 for January. April (4) is what I'm looking for.