Apart from the very early and the latest issues Enschedé blocks are the most difficult to identify because many of their print runs were experimental until they found a method of producing phosphor bands to satisfy the post office machinery.
You might find it helpful to read this Enschedé Definitives if you get into difficulty (opens in a new window or tab).
Of course the difficulty adds to the interest and they are the most popular blocks to collect with many varieties.