The back side of it looks pretty flat - perhaps it was buried face down, in two pieces. The Romans were pretty crafty about re-using things. Besides which, ev

The back side of it looks pretty flat - perhaps it was buried face down, in two pieces. The Romans were pretty crafty about re-using things. Besides which, ever been to/seen the streets of Pompeii? Flat, level surfaces for road construction only seemed important to the Romans as a side quest. They would fill the gaps with stone and mortar - perhaps that is the case here.