The Derby Silver Company (sometimes used the name Derby Silverplate Company), Birmingham, Connecticut, was founded in 1873. In 1898 they were one of the original firms that merged to form the International Silver Company. International Silver continued to use the trademarks of the original companies for many years.
That's probably EPNS in Gothic script which stands for ElectroPlated on Nickel Silver (silverplate). Your numbers 1614 and 1691 are just production or pattern numbers.
I have a hunch you both have the half circle mark (see attached scan). International started using this mark about 1921. Not sure how long they used it. The Derby factory was closed in 1933 in Birmingham and was consolidated with other plants in Meriden, Connecticut. Don't know if the Derby/International mark was used after that.
http://img143.imageshack.us/img143/787/scan10001kt1.jpgRegards,
Pat.