Here are some photos of the markings and other characteristics that show it was sent to Spain as aid from Russia.
Made in the USSR stamp that Spain added when these were exported to the US
1937 Date. All SCW Mosins will have dates from 1937 or earlier
Crude replacement hand guards are often found on SCW rifles. This one has screws and brass sheets added to reinforce it.
Here is a link to an excellent website on Spanish Civil War Mosins: http://scwmosin.weebly.com/index.html