It wasn't on mine either, but on advice given to me I called up the optometrist to ask and they gave it to me with no problem at all.
I've started keeping//tracking my prescription some time ago just for good measure.
Fortunately I was able to find it on a prescription from a previous provider.
I guess I was lucky to some extent, and I hope we're all able to find "better" providers.
However, where the optical centre of a lens is placed within a frame can vary from the PD a fair bit, depending on how the frames sit on your face, as the contact point of the frames do not always line up perfectly with the top of the nose bridge.
Also, an oft looked over measurement is the distance from the bottom of the frame to the centre of the lens, which is important with complex prescriptions and cannot be measured without looking at the frame on your face.
Having said this, with most low prescriptions the human tolerance is so high that it does not matter.