I have taken pics of the 'stones'. I remember reading somewhere that this happened in old glass along with faults which this has. In the Top ring of this is an indent or Mark that is in the last Pic below. It looks deliberate so as you mentioned Paul it does have a marker.It is at the 650ml point. I put the link in to ebay as it is the same as this but in a smaller version. Scotland seems to be where it came from. A stab in the dark but could it not be an early version of our 4 pint Jug that some pubs use. It seems that there would be enough for a number of measures for a group of people ??
I am seeing two amounts of ml into gill. 1ml is .0070 and .0085 these give a capacity of 4.55 gill (UK) and 5.525. Like I said, it makes no sense to me apart from the two differences in measures.