Wrothgar,
No, it's not particularly valuable- just a plain working man's pumpgun from years gone by. But it's a family heirloom, and for that reason alone you should value it highly and treat it with respect. It's a very robust design, and both the 520 and the 620 saw service as trench guns and riot guns in the US military.
The 620 is an evolution of the Stevens 520, which is essentially the same mechanical design. The 620 was a slave to fashion though, with its streamlined receiver. The 520 had the typical hump-back receiver profile that was a sort of trademark of its designer- someone no one here ever heard of before- John Moses Browning 8^).
Browning filed for the patent on what became the Stevens 520 on July 19, 1903, and sold the manufacturing rights to Stevens. The first 520 came on the market early in 1904. It had an odd double-hump receiver, and looked sort of like the receiver profile of the BAR, minus the rear sight.
Savage bought out Stevens in 1920. Tastes in shotguns had shifted to favor the streamlined look of the Remington Model 10 and the Winchester Model 12, so in 1927 Savage/Stevens introduced the Model 620. Mechanically it was the same as the old 520, but the receiver displayed the now popular streamlined look.
The 520/620 is a takedown design- the best such design IMHO ever available in US repeating shotgun history, except perhaps for the Burgess Folding Police Gun upon which it was based. To take it down:
1) Clear the gun to make sure both magazine and chamber are unloaded. Hammer should be cocked, and safey ON.
2) Open the bolt fully to the rear.
3) Grasp the ribbed section of the magazine tube and turn it clockwise (as viewed from the rear). As you turn the magazine tube, note that the keyed magazine tube base retracts forward, away from the receiver.
4) With the magazine tube base all the way forward, push down on top of the barrel's chamber while pushing up on the bottom of the receiver with the other hand. The barrel assembly should move freely down in the receiver about 1/4" until stopped by the action bar contacting the inside of the receiver. At this point the action bar will be separated from the bolt.
5) Withdraw the forearm forward until the action bar comes all the way out of the receiver.
6) Resume pushing straight down on the top of the chamber and straight up on the bottom of the receiver, being careful not to drop either half of the shotgun. The two parts should separate easily. While separated, be careful not to damage the ridges on the barrel assembly.
Reassemble carefully in reverse order.
That should be enough for normal cleaning. I would remind you again- this is a Browning design. That means LOTS of little fiddly bits best left in place in the gun.
IF you need to go further, I suggest you simply remove the buttstock from the receiver (take the buttstock mounting bolt out of the receiver tang, using a properly fitted screwdriver, then pull the buttstock off the tangs to the rear) and soak the entire receiver assembly in kerosene or diesel fuel for a few hours, using a safe (not glass) covered container in a properly ventilated place not exposed to fire or open flames. Then let it drain safely under the same conditions, changing its orientation from time to time until the residual liquid has all drained out, and relubricate sparingly. Then replace the buttstock.
The barrel/magazine/forearm assembly comes apart easily. Just remove the single machine screw holding the magazine end piece to the lug on the underside of the barrel- again, please use a properly fitted screwdriver. Be careful, because the magazine spring is under pressure behind the magazine end piece and if you let it get away from you, the magazine end piece will go flying. It'll be pretty obvious how it all comes apart and goes back together at that point.
Be good to this gun, don't get any weird ideas about detail stripping it, or refinshing anything. It has honest wear, put there by long use in the hands that were responsible for you being in this world- leave all that in place, please. That's all part of your heritage. Preserve it so you can pass it on to future generations without bunged up screw heads or other signs of kitchen table gun butchery. There's enough of that out there now as it is without adding more.
Enjoy it, it's a great gun, and you'll never get another one with this gun's history.
lpl