Jenkins allows the user to override the separator used when concurrent builds need more than one workspace. By default, the separator is "@", but this can be changed on the jenkins command line with:
-Dhudson.slaves.WorkspaceList=<some string>

This part of the plugin code is the culprit:
private String getConcurrentClientName(FilePath workspace, String p4Client) {
if (workspace != null) {
// Match @ followed by an integer at the end of the workspace path
Pattern p = Pattern.compile(".*@(
d+)$"); <<<<<<<<<<<<<<<<<<<<<< @ is hard coded, but this separator can be changed by the user
Matcher matcher = p.matcher(workspace.getRemote());
if (matcher.find())

{ p4Client += "_" + matcher.group(1); }

}
return p4Client;
}