[MarlandS]

When this model was introduced in 1931 the receivers were originally stamped "5000", this stamping was changed in 1941 to model "5100". The 5100 was the predecessor to the 311 and was produced from app. 1931 to 1948 (With no production for most of WW II) when they began stamping Model 311 on the frame / Early 311s will have both 5100 and 311 stamped on the frame , the closest listing to the original model is this:

MODEL 311 SxS
- 12, 16, 20 ga., or .410 bore, 3 in. chambers, double triggers, extractors, VR. Disc. 1989.
Grading 100% 98% 95% 90% 80% 70% 60%
$245 $205 $185 $150 $140 $125 $115

Last MSR was $309.
Add 30% for .410 bore.
Add 20% for 16 or 20 ga.

Other reference sources show a much higher value but prices in auction sites were in line with those posted

 

[MarlandS]

it's possible that it's a prototype, but more likely it's one of the earlier models. Stevens had somewhere around a dozen SxS shotguns on the market in their early years and many had no model number, the Springfield tradename was even more notorious for not having a model number..