Well, unfortunately it won’t fit your ‘78 750K due to the part number difference. Here's why:
The Joker website lists that brake strap as:
Honda part no: 43431-286-010
The part number cross reference shows that 43431-286-010 will only fit the following Honda models:
CB350 A MOTORCYCLE, JPN, VIN# CB350-1000001
CB350G A MOTORCYCLE, JPN, VIN# CB350-5000001
CB350K2 A MOTORCYCLE, JPN, VIN# CB350-2000001
CB350K3 A MOTORCYCLE, JPN, VIN# CB350-3000001
CB350K4 A MOTORCYCLE, JPN, VIN# CB350-4000001
CB500T A MOTORCYCLE, JPN, VIN# CB500T-1000001
CB500T A (76) MOTORCYCLE, JPN, VIN# CB500T-2000001
CB550F A MOTORCYCLE, JPN, VIN# CB550F-1109886
CB550F A (76) MOTORCYCLE, JPN, VIN# CB550F-2000003
CB750K A (76) MOTORCYCLE, JPN, VIN# CB750-2540001
CB750K0 A MOTORCYCLE, JPN, VIN# CB750-1000001
CB750K1 A MOTORCYCLE, JPN, VIN# CB750-1044650
CB750K2 A MOTORCYCLE, JPN, VIN# CB750-2000001
CB750K3 A MOTORCYCLE, JPN, VIN# CB750-2200001
CB750K4 A MOTORCYCLE, JPN, VIN# CB750-2300001
CB750K5 A MOTORCYCLE, JPN, VIN# CB750-2500001
CL350 A MOTORCYCLE, JPN, VIN# CL350-1000001
CL350K2 A MOTORCYCLE, JPN, VIN# CL350-2000001
CL350K3 A MOTORCYCLE, JPN, VIN# CL350-3000001
CL350K4 A MOTORCYCLE, JPN, VIN# CL350-4000001
CL350K5 A MOTORCYCLE, JPN, VIN# CL350-5000001
SL350 A MOTORCYCLE, JPN, VIN# SL350-1000001