Nachschub and H.Za.

[Spotswood]

Hi Guys,
I am looking into deliveries of vehicles but I do not have a clear understanding about Nachschub Ost and H.Za. Were they 2 different authorities or did nachschub work for H.Za. or was there something else as well? I have tried the search function and the internet but cannot find anything that clarifies it. Or is there a book out there that explains it?
Any help appreciated, regards Neill