1936 Baltimore City Directory, page 717:
Kopp Milton C (Helen) office boy Balto Assn of Commerce h1911 n Castle

1940 U.S. Census, Baltimore City, Maryland
8th Ward, Enumeration District 4-167, Sheet 1A
1526 Chapel Street, household, rents, $18
Kopp, Milton C., head, male, white, 25, married, not in school, highest level of education 8th grade, born in Maryland, living in same place in 1935, not on a farm, at work week of March 24–30, 42 hours worked that week, machinist for air plane factory, wage worker, 26 weeks worked in 1939, income $546 in 1939, no other income
Kopp, Helen E., wife, female, white, 26, married, not in school, highest level of education 8th grade, born in Maryland, living in same place in 1935, not on a farm, not at work, not seeking work, engaged in housework, no other income
Kopp, Helen D., daughter, female, white, 7, single, in school, highest level of education completed 1st grade, born in Maryland, living in same place in 1935, not on a farm, not at work
Kopp, Gladys A., daughter, female, white, 6, single, in school, highest level of education completed “0”, born in Maryland, living in same place in 1935, not on a farm, not at work
Kopp, Ronald M., son, male, white, 2, single