You have a Federal Signal Model 17 Beacon Ray, 12 VDC, the serial decodes to June, 1968. Your Model 17 has the large diameter dome that was also used on the 4-bulb Models 174, 175, and 176 Beacon Ray. Based on the scuffs and scratches on the dome, it is likely plastic, not glass, but blue domes are highly prized items for Beacon Ray collectors.