The challenge with saddle bag brackets is that they were never marked for what bike they fit. That was only a label on the outside of the box. Once they were out of the box, you can't tell. The best thing to do is to search eBay for as set the came off of the bike you have, or one that uses the same basic frame. With very rare exceptions, the bags themselves are all the same and will fit on any bike once the bracket is mounted. If you have some fabrication skill, it is not difficult to modify those brackets. I've done it a few time.
Likely Type 2 or 4 lowers would be closest. I put Craig's reference chart below for you to check.
Lower Guide
Type 1: Fits pre 1979 Honda GL1000s and 1977 CB400F
Type 2: Honda CB750A; 1977-8 CB750K; Suzuki GS400X; GS550E; GS750E, L & Yamaha XS11, Special
Type 3: 1977-8 Honda CB750F; 1978-79 Kawasaki KZ650; Custom, KZ900, KZ1000, Std & LTD, MKII, 79 Suz GS 850 - 1000, EL
Type 4: Honda CB 550F, CB550K, CB650, Kaw KZ750; Yamaha XS360, 400, 500, 650, 650 Special, 750 and 750 Special
Type 5: 1975-6 Honda CB750F, 750K
Type 6: 1977-79 BMW/7
Type 9: Honda CX500, 78, 79. 80.
Type 10: CB750 K, L ,F, 1979-80
