The "worm" is the squiggly, vertical line between the two birds, NOT the horizontal snake in the upper half of the circle. A quick way to tell the difference between the two varieties is to look at the worm: on the "Long Worm" variety, the worm is heads-up while on the "Short Worm" variety, the worm is upside-down. Both varieties share the same reverse.
Many (perhaps all) of the Short Worm examples have a raised die line connecting the S of SHILLING with the head of the bird on the left. They may also show a raised die chip just below the tail of the snake.