It is a hydraulic "Vickers unit" which was used on the Sperry upper and ball turret. There were two in each turret one for azimuth and one for elevation. The two units bolted onto a double ended 28 volt electric motor (one Vickers unit on each end) to make the power unit.
The Vickers units are pretty cool as they are very sensitive and you can control them easily from zero to full speed. The unit uses a variable displacement hydraulic pump to turn rotary mechanical power coming in from the constant speed electric motor into variable rotary mechanical power out to the turret gear boxes.
They aren't too rare as there are more of these Vickers units around than there are the turrets that they came from. When the turrets were scrapped often these units were saved and used for other projects.
Here is a Palleys ad for the pump from one of their catalogs:

The catalog is chock full of cool WWII aircraft parts and components. The pump is only $59.50 which is a lot better than the $1500 the guy wants in the eBay auction. In case you want to buy one from Palleys just call them up. Their phone number is Ludlow 8-1153. Sorry I couldn't find an email or web address on the catalog.