If I remember correctly, it was a cyanide pill. Issued to “Jedburgh” teams prior dropping into Nazi occupied Europe in WWII.
You’re right. After I did an Internet search, the answers came up. OSS = Operatives, Spies & Saboteurs, and the L pill was a rubber-coated, potassium cyanide (or other fast-acting poison) pill to bite in case of enemy capture.