Viol and Viole are generic names for string stops. The other names are synonyms given only by Wedgwood. While the tone of these stops varies considerably, most sources consider it to be of the Gamba class. According to Irwin, these stops may be imitative or unimitative, but Audsley maintains they are always unimitative. Adlung considers Viole to be a synonym for Viola, or a 4' Violdigamba. He also claims that the name Viol is sometimes used for the Offenflöte, and cites an example in the Marienkirche at Danzig (Gdansk, Poland) at 3' (2-2/3') pitch, but this is probably an anomaly.
Osiris contains a dozen examples of the unmodified name Viol, one each at 2' and 16', three at 4', and the rest at 8'. The name Viole is more common, with 38 examples at 8', 15 at 4', 4 at 16', and one each at 2' and 2-2/3'. The earliest known examples date from the late 1800's. No examples of the other names are known.