Daniel_J says

Triod Postnaja attempts to mimic the typefaces used to publish Old Church Slavonic service books prior to the 20th century. It also provides a range of Latin letters in the same style, sufficient for the needs of central, northern and western European languages.

Most OCS fonts that provide Latin ranges are problematic in the Latin lowercase, because they are not mimicking the uncial style; I have attempted to avoid this problem, especially in lowercase letters with ascenders or descenders.

I created many of the basic Cyrillic letterforms after eyeballing a very low-resolution image of the repertoire of Monotype Old Bulgarian.