Anybody know what the hole in the case back is for? Currently for sale on ebay, not mine.

 

The hole is to allow for water infiltration into the case.

 

Rudimentary Helium escape valve..

 

Those plugs are found on monocoque cases, and it is speculated that the hole allowed the factory to regulate the watch after casing. The plug looks to be factory sealed. See here for details:
http://watchguy.co.uk/service-and-repair-tissot-seastar-automatic-calibre-2770/

 

Yes this is for regulation after servicing.
You can screw a little tool to remove it easily that you can find in some sideral tool box.

In any case no helium valve.