Yes the internal one replaces 5th gears & holds the 2 shafts in place via a figure 8 looking shaft collar brace.
For most people a shot peened well built standard box will do well, next step in cost is PAR followed by Quaife then PPG.
Image below of 5th gear shaft collar brace, one in par kit & 1 for standard >
For most people a shot peened well built standard box will do well, next step in cost is PAR followed by Quaife then PPG.
Image below of 5th gear shaft collar brace, one in par kit & 1 for standard >