C5 does not fit any known numbering system (although the 'Alphabet' Series does use the number C5).
The glazes here look like Later Series. The shape resembles an un-numbered "Candle stick holder made from large ball" that appears in Later Series shape books near the end; see Later Series 197 for a sketch.
Height; 10 cm. Length of foot; 15 cm (left), 20 cm (right).
All unsigned. Cream example un-numbered. Bases of green pair incised C5.