The Spanish Civil War is a relatively un-remembered conflict here in the US, but it is a fascinating event for firearms enthusiasts because of the huge variety of arms used. Both sides were supplied by a variety of other sympathetic nations, including Germany, Italy, the Soviet Union, and Poland. The military aid supplies included both cutting edge material used to test new arms and tactics in a proxy war (such as German Stuka dive bombers and Soviet PE and PEM sniper rifles) as well as obsolete or obsolescent material sold to desperate groups who would accept anything they could find. The Soviet Union in particular used the Spain as a dumping-ground for all the miscellaneous small arms they had accumulated during and after WWI, sold to Spain for hard gold bullion.
Well, a reader named Leonard Heinz wrote up an extensive analysis of these small arms, you can read in PDF format via that link. His work is footnoted extensively, and he also created a sortable Excel spreadsheet of the different types of arms, their recipients, shipment dates, and ships used to deliver them. It’s excellent work, and great reading for anyone interested in the topic. Thanks, Leonard!
If you are interested in further research, the books Leonard references in his paper are:
Arms For Spain, by Gerald Howson
Mauser Military Rifles of the World , by Robert Ball
The Standard Catalog of Military Firearms, by Phillip Peterson
The Battle For Spain, by Antony Beevor
Between the Bullet and the Lie: American Volunteers in the Spanish Civil War, by Cecil Eby
The Republic Besieged, by Paul Preston and Ann Mackenzie
The Republican Army in the Spanish Civil War, by Michael Alpert