oki doki , got it sorted. I've been mis reading the number stamped on the back , it reads 393 not 398.... so, its part number 43100-393-781 and fits the following:
CB550F A (77) MOTORCYCLE, JPN, VIN# CB550F-2100001 TO CB550F-2114871
CB550K A (77) MOTORCYCLE, JPN, VIN# CB550K-2000007 TO CB550K-2015308
CB550K A (78) MOTORCYCLE, JPN, VIN# CB550K-2100001 TO CB550K-2122082
CB750A A (77) MOTORCYCLE, JPN, VIN# CB750A-7100001 TO CB750A-7110560
CB750K A (77) MOTORCYCLE, JPN, VIN# CB750-2700009 TO CB750-2729534
CB750K A (78) MOTORCYCLE, JPN, VIN# CB750-2800001