I should know this......  

Hmmm
There may have been something along these lines being sold on eBay before. In my opinion, it looks horrible.

good point
I'll just remove wide mounts and replace with the narrow mount configuration. The shocks are all really in bad shape anyway and should be removed and replaced.
Thanks woodywood. I almost thanked you ahead of time in the first message. ha!

I've not seen one before
... and I'd be surprised if Herman Miller ever made one. After all, they sold the chair with the seat and base per the customer's request. I can't see why they would want to reconfigure the shock mounts and change the base. They'd probably rather have sold them a new chair.
Maybe as woodywood suggested, someone sold an after-market component for those who wanted to change the base on their vintage shell.

i've only seen dowel bases...
i've only seen dowel bases for wide-mount shells on ebay UK.
Maybe you can make an adapter yourself? I'm thinking a bent metal slat with 2 slot holes.