This is a mounting spacer for a curved base Federal motor siren (60, 70, C, and Q series) on 1971-76 M-M and 1972-75 C/B 54" ambulances:
http://www.ebay.com/itm/1973-1976-R...ash=item3f5b167c87:g:EuAAAOSwuYVWoBB6&vxp=mtr
http://www.ebay.com/itm/1973-1976-R...ash=item3f5b167c87:g:EuAAAOSwuYVWoBB6&vxp=mtr