def get_ap_os_version(ap):
    """Parses os_version from AP data

    Removes model from start of os version if found.
    
    Args:
        ap (dict): ap data returned from FortiOS managed-ap REST endpoint
    
    Returns:
        str: OS version on 'unknown' id not detected
    """

    if 'os_version' not in ap:
        return 'unknown'
    return ap['os_version'].split('-', 1)[-1]