Metrological standard measure for eggs

Metrological standard measure for eggs;

The inscription reads: “Association of the Exporters of eggs and poultry in Bulgaria” with dimension mark above “40 75 m/m". The measurement with a caliper gave exactly 40.75 mm, which is incredible, having in mind how much rust I’ve got to remove.
The specific sign Yat (http://upload.wikimedia.org/wikipedia/commons/thumb/4/48/Cyrillic_letter_Yat.svg/150px-Cyrillic_letter_Yat.svg.png), removed with communist’s orthography reform gives me a clue it has been in use at least before 1945, but I will not be surprised if it is from the begining of the XX century, most likely c. 1920-1939;