(By Jakob Lesage): "This question concerns a phonologically bound verbal marker that signals that the verb has an instrument. In some languages, applicative markers may be used to topicalize, focalize or background the instrument. This type of marking also counts for this feature. The marker could be analyzed in the language-specific case as voice, indexing or a ‘bound pronoun’. The marker does not have to index the instrument."
Instrumental indexing without the presence of an instrumental applicative marker seems to be extremely rare if it exists at all. (It may be that it is included in Grambank only for the purposes of parallelism with GB103, which covers benefactive person indexes or benefactive applicative markers.)