I just bought a 400-Watt Ballast from EBay and it has an odd female receptacle. The seller hasn't been much help. The 3 holes are inline BUT off center.

I've attached the image of the receptacle.

I like to connect it to a E-39 Mogul style 400W MH bulb but don't know what connector or prongs to tap.
Any help is highly appreciated?

Over 30 years in the trade and specialized in lighting, and never saw a device such as what you have.

Is there any markings on it at all?

Do you need a male and female attachment plug connection, or can you just eliminate and hard wire?

Over 30 years in the trade and specialized in lighting, and never saw a device such as what you have.100% agree.

I bet that is some import fixture that was not designed to be used in North America. That or it is a proprietary plug/socket setup for that fixture or company.

Why is there a female socket tail? Is it meant to be daisy chained with other fixtures?

It's the standard plug for sunlight supply ballasts. It's the output that goes to the lamp socket. You can buy adapters for them. It is made 100% FOR north America, actually it's American made. Just bastard so you have to buy their reflectors or adapters...

I know this is an old thread but thank you for this information.

Yep, correct, made by Sunlight Supply:

Sunlight Supply, Inc - Horticulture, Lighting, and Gardening Wholesale - Female Receptacle for Lamp Cord (http://www.sunlightsupply.com/p-11573-female-receptacle-for-lamp-cord.aspx)

The name should have been found on the fixture.