Hogarth’s Gin Lane published c1760 shows hanging signs of almost this shape. In 1790 capacity standards were likely the Old Ale Measure (known as 1688 Customary Ale measure and of 20.62 fl ozs to the pint). And also the Queen Anne Wine measure of 1707 (16.69 fl ozs to the pint). The imperial standard of 20 fl ozs to the pint came into force in 1826.
The Old Ale Standard was so near to Imperial (being just a bit more) that the inspectors from 1826 accepted and ‘sealed’ them. Only Types 1 and Types 2 are likely to be of pre-imperial capacity.