What fields are available when filtering on /Party?

I can filter /Party using a number of fields including CustomerTypeCode, but when I try MajorKey it fails.

Asi.Soa.Core.Exceptions.EntityServiceException`1[Asi.Soa.Core.DataContracts.Faults.ArgumentFault]: ArgumentFault: In the query, MajorKey is not a known property name.

/api/metadata/Party doesn't seem to expose a list of fields I can filter on, and both CustomerTypeCode and MajorKey are in the same AdditionalAttributes section. Is there a list somewhere of what I can filter on, or is there a different syntax/variable name I should be using for the Major Key field?

Thanks.