eBay Photo: Fair Use
Here is a film from England for sale on eBay showing how an exposed film can was marked. Don't know what the markings after 'Rated @ 80' means. 80 is the ASA, maybe the rest is something about a neutral density filter?
Artist's tape, white or colored gaffers / spike tape or medical tape is good for marking cans.
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DDTJR Small Gauge Film Archive