Generally, (not a set in stone rule as someone will always be ready to show you a gun that doesn't fit the categorys)....

The '520' is the early receiver style with the steps to the profile. These have a flat spring mainspring and trigger spring mechanism. Trigger guard safety.
(The entire trigger mechanism inside reminds you of an early A-5 when you first see it)


The 520A is the later receiver style with the straight line top and bottom profile lines. Top tang safety.
These will have a coil spring trigger and mainspring mechanism.
I've always believed these to be 1930's manufacture, but I don't have any paperwork to prove it at hand. I may have read it somewhere but that doesn't prove anything.

The 520-30 was the WW2 Military designated Stevens Riot & Trench Gun 12ga marking on the receiver. (Coil springs, top tang safety)
But I had a sporting gun (30" Full) Stevens marked 520-30 that I sold not long ago. So there's one of those that doesn't quite fit the category.

I also recall reading that the 520A sporting gun had (just) been discontinued before we entered WW2. Needing shotguns for the War effort, Savage brought the machinery back out of storage and set it up to produce the 520-30 for Gov't Contract.
620's were also made to fill the contract.
The folks & forums that collect the US Military Trench Shotguns would probably have info on this as well as some serial number range for at least the War time manufactured guns.

I think the 520(30) again went away after the War. The 620 continued on.

Wards & Sears had their own Models designations stamped on them other than the 520 series. Sears used the catalog number as usual for them.
'Ranger' & 'Western Field' were trade names used for those firms.
It seems as though I had a Sears/'J.C. Higgins' marked 12ga also. But that would imply post WW2 mfg if that J.C.Higgins name was on it I think(?).
I've owned way too many of these and there was always differences however small even in guns seemingly made during the same time periods
I don't believe anyone will ever sort these out.

Speaking of how mfg'rs were looking at others products for ideas,,,I have a Meriden pump 12ga.
A simple but nice handling light weight shotgun. Not a lot of them around.

The bolt/sear/striker assembly and the takedown adjustment features are very similar to the Remington Model 10 shotgun.
Meriden pumps were made in only one year,,1918 IIRC.
The bbl is marked 'Krupp Fluid Steel'.
The op-rod is marked 'Savage Patent'.