Daft question - some more pricey v brakes or cantis are distinguished as front or rear. So, what's the difference between front and rear? The only thing I can think of is pad orientation for cartridge pads - and surely those can be swapped over if necessary?
it is the noodles - the front has a 130 deg arc to allow the uk standard right hand front lever to be connected by a smooth curve of cable wihtout excess bending i.e. the noodle points towards the left side of the bike (looking down on the bike from the saddle) so that the cable can be formed into a neat curve from both the lever and the noodle. On the rear (and if you want your front brake to be on the left lever) this isn't an issue so the noodle is about 90 degrees and the cable pops straight into it. Cheap brakes which don't make this distinction end up with a kinked cable on the front with normal right hand front brake lever. No idea why cantis should be different front and rear though!
...spare noodles being available quite cheaply thus allowing you to use front on the back and vice versa. I've ended up using a front noodle on the back as it gives better 'cable line' due to a funny cable stop location.