The 520's patent was filed 10 July 1903 and sold by Browning to Stevens. The shotguns went into production in 1904. This was Browning's second pump shotgun patent, the first was the Winchester 1893, which evolved into the 1897. Like Dave said, unless you're really good, leave the inside of the receiver on this one to a gunsmith.
The 520 stayed in production (with various modifications to the safety location, action release location and receiver shape) until 1932. Older 520s have a 'double hump' receiver that reminds me a lot of the BAR, newer ones, the 520-30s, are 'square backs' like the current H&R Pardner Pump.
Neat shotguns... usually cheaper with MonkeyWard markings, but 150 does not seem too bad for one in good mechanical shape.
If you want the same basic action and takedown system in a newer gun, look for a Stevens 620 with the streamlined receiver.