Theo Wanne Glass Gauge for Mouthpiece Measuring - Traditional Glass Gauge for Mouthpiece Measuring - Traditional
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Description
When reading mouthpiece facing numbers, this is the gauge that is usually referred to. It shows the actual mm length times 2. For example, if the facing is 20 mm, the gauge will read 40. If in doubt of which gauge to buy, buy this one, as most facings are given with this system.

