Congratulations
My second baby was posterior and I had the 'numb' feeling in the abdomen that you describe, for some reason nobody picked up on it until her head was delivered and the whole time I'd been freaking out that something was seriously wrong because I couldn't push properly and was completely devoid of sensation at the front of my body.
My first baby was also a back labour, born facing the right way, but I still had sensation in my tummy - the pain, however, was all in my back. So... I guess no, a 'back labour' doesn't necessarily mean a posterior baby, but if there's very little/no sensation in the front, then chances are it is, kwim?
Sorry, that wasn't helpful at all, was it lol? :/