28  (Early Series)    'Ginger-jar' vase
Does this shape appear in other series?  Yes - Later Series 24.
Curiously it also appears again in the Early Series as Early Series 42.  We can only speculate as to the reason, but probably Premier realised their mistake in having two shapes numbered 28 and promptly corrected it by assigning this one a new number, 42.  Almost all Early Series examples of this shape are numbered 42 whereas 28 is rare.

28  (Early Series)    TWO SHAPES
:  Having two shapes numbered 28 probably happened by mistake and was promptly rectified by giving one of them a new number.

C.V.Jones Antiques & Art Gallery,
Ballarat, Vic.
Shape book 'E'

Davidson Auctions
These two pieces have similar glazes and are quite possibly from the same firing batch - maybe the only batch produced with this number.
28  (Early Series)    Globe-shaped vase with conic flared lip
Does this shape appear in other series?  No, but there are similar early un-numbered examples.
Early Series 20  looks rather similar.

Height;  10.5 - 12 cm
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Height; 15 18 cm.

28 L
Height; 19 cm.

Collection; 'W'