The only other instance of this particular neume that comes to mind is the offertory Ave Maria on the first syllable of the word "ventris." In both cases, the corresponding note in St. Gall is a virga with episema.
The thing with the Paleography is that the signs are a mixture of chironomic gestures, copyists' shorthand, and just what happened to be the simplest way of making distinctions between duration and pitch.
What you have in Laon is simply two short notes of the same pitch followed by two quickly descending notes. The same in St. Gall: the tractulus ( _ as opposed to . ) denotes notes of lower pitch. There is nothing particularly chironomic about this form.
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