The solutions of the main problems associated with the formation of digital mathematical collections from documents published in the pre-digital period are presented – such collections are designated in the work as retro collections. Algorithms for creating a meta description of retro collections based on the analysis of the structure of mathematical documents and the use of software tools for extracting metadata are given. The description of retro-collections formed using the developed algorithms and included in the metadata factory of the digital mathematical library Lobachevskii-DML is given. The schemes for the formation of metadata and methods for normalizing the extracted metadata in accordance with the schemes and requirements of the integrating mathematical libraries are indicated.