Do you know what this is?
This is a replacement Coleman Lantern mantel; replaced when the old one gets damaged or burnt out. I didn't use them for that purpose.
The mantel exhibits a peculiar property in that it is slightly radioactive. Because high radioactive sources were tightly controlled and inventoried in my work I needed a quick OP/NO-OP method for checking radiation monitors such as a Gamma-Ray tool. Potting two of these together and placing them near the GR detector would quickly tell me that it worked by registering the counts. Also, the same potted mantels would sub for a source to troubleshoot Geiger-Mueller bundles.
In any case the mantels were NOT for calibration; test sources were used for that and those were tightly controlled.
Another "source" that oil well logging companies use is sand. Not just any sand but sand gathered from the beaches of Guarapari, Brazil and potted in a flexible blanket.