I've been looking through these and found some interesting varieties, with some help from Michel
500T Mark. Note the thick edge below the bottom left post horn.

1M Mark. Note the thick line underneath the top right post horn.

1M Mark. Note the joined lines underneath the top right post horn on this rather poor stamp.

1M Mark. Broken sprung at 9 o'clock.

2M Mark. Splodge over lower right post horn.
