Legalese: case per case Specification

[Grablevskij]

In my textbook an example is given.

Payment is to be effected in US dollars within... days of receipt by the Buyers of the following documents:
a) Case per case Specification in duplicate.

The textbook is by an author whose native language is not English.

I have some doubts about "case per case". Maybe it is a mistake. I consult webseter dictionary
http://www.merriam-webster.com/dictionary/per and can't find the suitable meaning of per in this case.

Anyway, I can't understand the grammar here and would be grateful if you help me.

[BobK]

It's not 'per case'. it's 'per [conforming with, constructed according to] case specification'. That is, the case has to be of the right sort.

b