TW,
Chequered front triggers are frequently seen on double rifles - especially if they recoil somewhat. It was common practice to fire the right barrel (front trigger) first so it was that trigger which was chequered to help prevent the finger slipping onto the rear trigger (on a gun that really is going to move around - like a 8 bore - I'd be inclined to pull the rear trigger first no matter how 'non-slip' the front one was).

It wasn’t done to differentiate one trigger/barrel from another – in any case triggers are so differently shaped that it would be difficult not to know which one you had your finger on!