How to explain the coefficient of Hill 48 yield criterion?

Can someone help me to explain the derivation of the coefficent ?

In the paper "A theory of the yielding and plastic flow of anisotropic metals", the Hill 48 yield criterion:

The quadratic Hill yield criterion for thin rolled plates (plane stress conditions) can be expressed as:

However, in the book of "塑性・粘弾性" link:http://www.worldcat.org/title/sosei-nendansei/oclc/047515845

the Hill'syield criterion was described as below.

Thanks.