To me it looks like a medicine of some sort. Probably around the 20s or 30s. Without any markings it's hard, if not impossible, to narrow it down beyond that without finding one with the paper label still on it. Although it's odd that there are no seams. Could you post a closer picture at the neck/lip?
I had never heard of Rubifoam for the teeth, so I googled it. From what I saw, they are all clear and more rectangular than square. The shoulders slope similarly though.