CS5 Master Collection Mac — genuine serialized licence, but Adobe registration rejects serial - please assist

Dear Adobe Legacy Experts and Community,

I have a genuine Adobe Creative Suite 5 Master Collection perpetual licence for Mac, originally purchased and installed from the physical CS5 DVD set.

I still have a fully working installation on an older Mac. The installation contains Adobe's SWIDTAG licensing file identifying:

Adobe Creative Suite 5 Master Collection
MasterCollection-CS5-Mac-GM-MUL
activation_status: serialized

The SWIDTAG contains my 24-digit Master Collection serial number.

However, when I enter this serial in my current Adobe account under Products → Register a product, Adobe returns:

"The provided serial number is invalid."

I have verified the serial directly from the existing licensed installation.

I need to reinstall CS5 Master Collection on another Mac. Adobe's current documentation says registered CS5 products can be downloaded from an Adobe account, but I cannot register this legitimate licence.

Can Adobe provide the official CS5 Master Collection Mac installer, or otherwise associate this existing licence with my Adobe account?

I am specifically looking for an official Adobe download, not a third-party installer.

Many thanks for your time and consideration… I look forward to your replies