My earlier post was done in haste, as I was getting ready to leave for work and had no time to elaborate.
This piece dates from about 1897, when Harry Northwood was just getting out some of his initial lines from the plant at Indiana, Pa. The butter base is pressed, but the cover is a blown/pressed combination, the dome having been blown upside down onto the previously pressed knob (this operation was called "hokey pokey" by glassworkers later when used for stemware). The original advert in "Harry Northwood: The Early Years" shows the table set and water set as well as s/p shaker, syrup jug, cruet, and celery holder. These pieces are hard to find, but I finally located some and had them photographed for inclusion in "Dugan/Diamond: The Story of Indiana, Pennsylvania, Glass" (p. 77). Since that time, I've been able to purchase items for donation to the Oglebay Institute's Carriage House Glass Museum, which has the most comprehensive Harry Northwood glass collection in the world and is well worth a visit.
Incidentally, these moulds were also used for Northwood's Venetian and Opaline Brocade lines in 1898-1900.