Spotlights were a common accessory of the period. A right side spotlight is less common, but not that unusual. The right side spotlight will likely have mounting base that is unique to the right side. The correct spotlight for a GM car of the period is the Guide S-16. The S-16 spotlight has a cap over the bulb that will be marked as "Guide" or "GM". The cap marked as GM is more desirable. The Guide spotlight right side base has a casting number of 5936476.
Guide spotlights sold through Wards, Sears, etc. are made by BLC (Brown-Lipe-Chapin), a GM division. The BLC lights are the same as the Guide lights, except that they have light tan plastic parts and a different bulb cap; the Guide spotlights have ivory colored plastic parts. BLC lights may also have been dealer installed, rather than the Guide spotlights.