it is indeed - the word written as Bristol with what seems to be a dremel type tool. The only piece I have from the 'Bristol Blue factory' is a milk jug, and without that signature there could be confusion since the handle is applied in pre 1860 style and the blue is very good with a snapped pontil - although as you'd imagine it lacks appropriate wear for a genuine period piece.
On your rummers I'm seeing snapped pontils and what appears to be plenty of wear on the feet, quite convincing features, plus I'm thinking these have possibly one of the rarer rummer bowl shapes of what looks like a double ogee.
However, have to remember that Thomas Webb produced a bristol blue range in the late 1970's (although all the pieces of theirs I have show a ground/polished pontil depression) - although from memory not all Webb material is signed. Much earlier in the C20 Hill Ouston were also offering period glass (including rummers), and they certainly offered blue, but I don't know if their pieces carry any sort of signature. When flicked, do yours produce a good 'ring'?
It is often suggested that one of the clues for testing for age in glasses is to compare the diameter of the foot with the same dimension of the rim - the theory being that if the foot is the same, or exceeds that of the rim, then it should be period.
Unfortunately, foot wear is not guarantee of age. Hajdamach quotes a source who relates that in Stourbridge a retired glassmaker was actually paid to sit on the front doorstep of the factory and grind the feet of wineglasses to produce the necessary 'period' wear!!
Sorry not to be of help, but nonetheless nice glasses, and if you didn't pay much then you have a bargain.