It's a newer design. Used on push clutch 5 speed gearboxes circa 2006 onwards. (in Australia, to my knowledge) May have appeared earlier in the USA market as you had EJ25 turbo engines before we did here. Very early WRX boxes had no brake on reverse gear and were difficult to select and often crunched on reverse selection. Circa 1996 Subaru revised the design to incorporate the reverse brake on the back of 5th gear. It applies a brake to the mainshaft as reverse is selected, it does not 'synchronise' reverse gear, it just slows the mainshaft to prevent crunching. I don't know why they have deleted the brake from newer versions, but the synchro and hub is definately revised and appears to be more robust. As always, use extreme care swapping parts between transmissions of differing eras, something will always catch you out.