Stevens (?) 520-30 Trench Gun

[mtrpdw]

I've had this Stevens (?) 520-30 "Trench Gun" for over 20 years. Would like opinions on what it actually is:

- S/N: 40xx8. Barrel assembly and receiver have same serial number. No s/n on stock.
- No "520-30" markings/stampings on left side of receiver, or anywhere else.
- Left side of barrel, ahead of receiver, only marked/stamped with "Proof Tested - 12 Gauge, - 2-3/4
Inch Chamber -".
- No "J. Stevens Arms Company" markings/stampings anywhere.
- No Choke markings/stampings anywhere.
- No U.S. ordnance or military markings anywhere.
- 20" barrel with what appears to be a factory crown.
- Bead sight hole has been filled in - located 3/8" back from muzzle.
- Three (3) transverse grooves milled into bottom of barrel to accept Heat Shield screws.
- Transverse grooves appear professionally machined into barrel.
- Bayonet Ring Lug not flush with muzzle of barrel - appears this was done to protect/cover
filled in bead sight hole - so transverse grooves milled in accordingly.
- Left side of Bayonet Lug part of Heat Shield is "S" stamped.
- Front of trigger guard has "M" stamp.
- Just ahead of "M" stamp, at bevelled loading ramp, is small "S" stamp.
- Entire shotgun is Parkerized - it is definitely an older finish and has some
discoloration/blotching due to aging.
- On left side of receiver, above trigger guard, are the hand engraved letters "CD", inside a
hand engraved triangle. Presumably, this stands for "Civil Defense". It appears that some
"yahoo" in the past took steel wool to the area of the "CD" engraving.
- Stock, Buttplate, and Swivel appear right, but no markings whatsoever.

Thoughts, comments...any ideas on what this may have been or is?

Cheers!