The 1840 issue has: - no white lines below 'POSTAGE' or above 'TWO PENCE,' - no perforations, - stars and no check letters in the top corners, and - no plate number engraved in the vertical borders.
The white lines were added in 1841, to help post office staff discern between the one penny and two pence stamps under bad lighting conditions. Later stamps were perforated. Finally, the stamps had the check letters replacing the stars in the top corner in reversed order and the plate number in the vertical borders.
These 2d blue stamps come in three completely different designs - the perforation did not constitute a change in design. Plate numbers, therefore, are not continuous across these designs,