Question: When did they start putting security strips in money?
Disambiguated questions and answers: question: When did they start putting a plastic security strip in US $20 bills?
When did they start putting a metal security strip in US $100 bills?
When did they start putting a security strip in US $5 bills?
When did they start putting a security strip in US $50 bills?
When did they start putting a security strip in US $10 bills?
When did they start putting a "3D security ribbon" in US $100 bills?
answer: 1992
1990
November 1994
November 1991
July 1992
October 2013
Natural Questions answer: 1990 (from the Wikipedia article "United States one hundred-dollar bill")