I don't think those circles should be construed as a maker's mark of any kind. These are likely part of the mould construction to allow the piece to "relieve" from the mould after pressing is complete.
We have a sugar basin and cream jug in this same pattern, and both have these circles on the undersides. I wonder if the cream jug and this "tankard" were made from the same mould. Perhaps the winner of the tankard will post careful measurements, and I can do the same with the cream jug.