def report_numbers2marc(self, key, value):
    """Populate the ``037`` MARC field."""
    def _get_mangled_source(source):
        if source == 'arXiv':
            return 'arXiv:reportnumber'
        return source

    source = _get_mangled_source(value.get('source'))

    if value.get('hidden'):
        return {
            '9': source,
            'z': value.get('value'),
        }

    return {
        '9': source,
        'a': value.get('value'),
    }
Populate the ``037`` MARC field.