Here is a result of rest request to retrieve version :

{ "self": "http://machcw.local:2990/jira/rest/api/2/version/10100", "id": "10100", "description": "version 1", "name": "ver1", "archived": false, "released": false, "startDate": "2013-07-09", "releaseDate": "2013-07-31", "overdue": false, "userStartDate": "09/七月/13", "userReleaseDate": "31/七月/13" }

But in the server side code, it does have a project field which is not assigned value. In the final json response, the project is lost.
com.atlassian.jira.rest.v2.issue.version.VersionBean
@XmlElement
private String project;