[updated:LAST EDITED ON Jan-19-05 AT 05:25 PM (CST)]According to '61 sales literature(other years probably similar, but not necessarily the same)There were four "custom equipment groups" Custom appearance option- silver anodized aluminum grille, stainless windshield reveal, steering wheel with chrome horm ring, chrome trimmed inst. panel knobs, two tone interior door panel treatment.
Custom Comfort option-All the above with..left side arm rest, right side sun visor, chrome cig lighter, extra foam padding on seat,extra cab insulation, left hand outside key lock.
Custom Chrome option-All the above with..Chrome plated bumpers, and chrome plated hub caps.
Custom side moulding-All the above with..Bright body side moulding for fleetside pickups only.
Thats as definative an answer as I can give, others may have more info on other years, etc.
As far as the "Custom" script in the middle, those are for the '64 and later trucks, and are of aluminum, the '60-'63 trucks had the "Custom" script at the bottom, and are made of stainless. You undoubtedly have the trim from an earlier truck installed on your '66.