This button is a press-molded wafer. Back marks are uncommon in casein buttons.
Casein identification guide: casein is a thermoset semi-synthetic polymer made from milk, hardened in formaldehyde
7. Many pearlized caseins, made by mixing fish scales into the plastic batch.
source: Button Materials A-Z: Identification Guide
by Jocelyn Howells
The above is based on the National Button Society's Classification System.
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