The Server companion products for Net Express are listed under product name Server for COBOL. If you have current maintenance for the Server 5.1 product then you should see it listed. Only the Server for COBOL wrappacks are available for download from microfocus.com. These are the .msp files that can be installed over the base release product.
In order to get a link to download the full base release of the Server 5.1 product you will have to contact your Account Manager or open up a support incident with Customer Care.
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