Unable to assign subscription to any version of RHEL 6

First, if this went to the wrong place it is because there is no topic for just plain old Red Hat Enterprise Linux for x64.

I have struggled all day to get a subscription assigned to a registered RHEL 6 server. I have already read everything I can find here on the subject and none of the answers addressed this problem. I have tried up to 6.10 although I need 6.1 to match my client's version of the server. We know they should update, that is what I am tasked with doing.

I have built and rebuilt the servers many times and have managed to successfully register all of them - eventually - but have not been able to get updates on any of them.

I have multiple available subscriptions and all should be eligible for RHEL of any supported version. Supported may be the key word there.

I have successfully registered, subscribed, deleted, registered, and resubscribed at least a half dozen RHEL 7 machines. I have plenty of available allocations. I could get updates on every one of them. I know the required steps by heart now. I'll probably have nightmares about them.

I have tried the same steps with RHEL 6 but all it ever says is: "Installed products are not covered by attached subscriptions. Updates cannot be received."

The subscriptions allow me to download versions going back to RHEL 4. One would think that if it is downloadable it should be possible to assign a subscription to it, especially if the repo is still active.

I am taking over for a developer who left our project and talked to him at length today. He has a running 6.10 server and was able to pull updates today, so I know that is still possible. It's the registration process that isn't working, not the RHEL 6 repos.

I have run all the subscription manager commands suggested here, but nothing has worked. I have worked on this for over six hours now and nothing I have tried, no suggestions I have found here, nor anything found on Internet have worked.

Has Red Hat pulled support for attaching subscriptions to RHEL 6?