This and all the other Finnish Mosins are detailed in my new book “Forged in Snow”:
https://www.kickstarter.com/projects/headstamp/forged-in-snow?ref=b8ft1r
In the 1920s the Finnish Defense Forces wanted a reduced-power training rifle to allow low-cost marksmanship practice and to allow shooting on small ranges. After investigating drop-in rimfire conversions, they decided to make dedicated .22 rimfire m/91 Mosins. A contract was given to the ATV-1 armory to make 1,500 barrels using the Salerno process; inserting .22 caliber liners into drilled-out m/91 barrels. These barrel were paired with a rimfire conversion of the bolt, and the result was the m/91 pienoiskivääri (“miniature rifle”).
Production ended in 1927 because of a scandal over the serviceability of Salerno process 7.62x54R rifles, and a second contract was given to Tikkakoski in 1929 for an addition .22 rimfire barrels. In total, about 2,500 of these rifles were made by the early 1930s. They were typically issued in batches of 3-5 per regiment, and used regularly until World War Two. In addition, a .22 rimfire version of the m/27 rifle was also produced, although not in sufficient quantities to replace the m/91 in this role.
During the Winter War Finland captured a huge amount of Soviet 7.62x54R ammunition, and also developed its own reduced-power loading. This made the .22 rimfire training rifles unnecessary, and they were sold off as surplus shortly after the end of the Continuation War. Most went to Defense Forces personnel and to film and theater companies to use as props. They were never sold in bulk to importers like InterArms, and so they are very scarce in the Unites States today.
Video on the EL-24 .22RF Kit for a Steyr M95:
Post WW1 most countries explored and/or actively developed .22 L R training rifles based on full-size military rifles or, like the U S Army contracted rifles from major manufacturers for use as training rifles. Germany had several models made by Mauser, USA had Springfield M1922 and Savage Arms rifles, Britain modified Lee Enfields for the purpose. Kudos to the Finns for a thoughtful approach to the situation.