Albert Vest asked about old NEN "sim" sources.
According to Ron Brown at DuPont/Merk (aka DuPont/NEN, aka NEN . . .) "Sim"
sources were indeed long-lived sources used to simulate short-lived
sources. For those in question: the sim In-113m source was Sn-113 [1 uCi
for the rod source {NES 208S}; 5 mCi for the glass vial], the sim Tc-99m
source was Co-57 [typically 5 mCi for the glass vial].
Hope this helps.