With the end of the war, most of the Mosin rifles were sent back to the arsenal for refurbishment, and then crated. Some were sent to allies as military aid. Most ended up in storage for WW3. With the easing of import laws to the US for Curios and Relics in the 1980s, and more importantly the collapse of the Spviet Union, these weapons began to find their way to the American market in cast quantities beginning in the 1990s.

So to answer your question directly, most likely it sat in a storage facility for 50+ years.