Shown in Davidson’s catalogues of their unregistered designs 1880-1890 as Davidson pattern #221.
An uncommon pattern. I have only come across examples in clear glass previously, though Slack shows an example in brown marbled glass in Fig. 53 on page 75 of ‘English Pressed Glass 1830-1900’. Your purple marbled vase is really nice and, as you say, quite rare.
Fred.