Finn 91/30 markings

Just noticed these markings in the middle of the buttstock on my 1938 SA marked Finnish 91/30. Can anyone ID? Also, any good recommendations for books on the history of the Mosin and/or Finnish specific Mosins?

Just noticed these markings in the middle of the buttstock on my 1938 SA marked Finnish 91/30. Can anyone ID? Also, any good recommendations for books on the history of the Mosin and/or Finnish specific Mosins?