numbered as you would expect, but to have alphabetical versions.
but they neither have more than 26 versions nor minor versions.
way, but I don't know where to start.
I fear it won't be possible to address this properly on the Repository tier. Version numbers are expected to consist of numerical values separated by dots. You could add a custom property of your own to track the Version number and keep the Alfresco internal version. Then you'd Need to customize the standard name metadata renderer in the document list of Share as well as the node Header and version history components in the detail view.
The simplest solution I can see at the moment involves configuring a custom version Label provider that creates version labels consisting of only one number ranging from 1 - 26 (and above if they should start to have more Versions) regardless of the type of version in Alfresco Jargon. And convince the client that a single number is just as good as a single letter.