"Earlier scribes tended to write the abbreviation more like a long 'z'."
These are actually two different Latin scribal abbreviations for -us (ꝫ and ꝰ) that could be used at the same time by the same scribe (but ꝫ is less common than ꝰ for -us), not the same written by different scribes at different times. The one that looks like a z or 3 (ꝫ) stood for -m and -et, -ost, -us, also qꝫ is -que: very ambiguous, but easy to disambiguate if you know which Latin words exist and are grammatically possible in the context.
ꝰ could have abbreviated something else in Catalan, I don't know. It's certainly not a "g" (written more like "8").
These are actually two different Latin scribal abbreviations for -us (ꝫ and ꝰ) that could be used at the same time by the same scribe (but ꝫ is less common than ꝰ for -us), not the same written by different scribes at different times. The one that looks like a z or 3 (ꝫ) stood for -m and -et, -ost, -us, also qꝫ is -que: very ambiguous, but easy to disambiguate if you know which Latin words exist and are grammatically possible in the context.
ꝰ could have abbreviated something else in Catalan, I don't know. It's certainly not a "g" (written more like "8").
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