For specific questions on the A5, it's history, repairs etc. with guys that are really into them try this website:
http://www.shotgunworld.com/bbs/viewforum.php?f=53
A little background on the history
The Auto-5 was the first working semiautomatic shotgun design. John Moses Browning considered it to be his greatest design, and he had a bunch including among other things the Colt .45, Winchester 97, the BAR, and M2 .50 Cal machinegun. It is a long recoil design, using the kinetic energy of recoil to power the operation of the gun. It is not gas operated in any way (as mentioned in an earlier post) As the A5 was originally developed in the era of black powder gas operation would have problematic. It is best known for remaining operational despite hard use, neglect and adverse conditions of temp, dirt, water or ice.
The Auto-5 was first marketed in 1903 and was manufactured for Browning by Fabrique Nationale in Liege Belgium. The gun was in continuous production for almost 100 years, with a couple of timeouts for world wars and the Nazi occipation of Belgium. The gun was so good that it saw almost no changes in those 100 years. The principle changes were the addition of a Magazine cut off, the 2 piece lifter and a cross block safety. It was produced in 12, 16 and 20 ga, with regular and light versions and Mag, (3" chamber) in both 12 and 20 ga.
The manufacturing was moved from Belgium (FN) to Japan (Miroku) in the mid 70's. Around 2 1/2 million A5's were made. In addition the Remington Model 11 and Savage 720 were based on the John Browning patents.
Your gun was known as the Mag Twelve and was introduced in 1958. It remained in production until the Auto-5 was discontinued in the late '90s
Enjoy it, it is a timeless classic.
Jeff