They are listed in the Scott Specialized Catalog of US Stamps under "Consular Service Fee Stamps" and would be either an RK27 (if perf. 10) valued at $42.50 or an RK34 (perf. 11) valued at $3.75.
For more information and explanation about their usage (which was discontinued in 1955), checkout this link:
http://www.ericjackson.com/lesher5.htm