Photoclubalpha

Take a look at the pictures below.

There's a small rubber insert on the battery door that is clearly intended as a means to drive external bolt-on battery power into the camera with the main battery door closed. So they obviously planned to make a kind of a large bolt-on battery for the 55/33.

Not sure it could be a proper battery grip, as there seems to be no proper interface to communicate the external signalling unless they use the battery contacts for some kind of 'USB' connection.

But a large battery sure was on the cards...

That's for the external power supply, which is similar to Canon - a dummy battery with an AC adaptor. Canon just use a ribbon cable which shoves through a weak area in the seal of the battery compartment. Sony decided to fit a proper rubber sealed hatch.

David

Sure that is for an external power supply. The matter of fact is that the particular arrangement is far more elaborate than a simple external power socket. And there must have been a particular purpose for doing so, no matter what Canon did or not.

And the only sane version is ...

It's for the AC adaptor AC-PW20.
It sure is handy when you're indoor. My son uses it.

Sure that's used for an external power supply.
But if the intention were to have an AC adapter connection only, a simple socket (as used on a700, e.g.) would suffice much better.