Western Field SB30A (520) U.S. Marked??

[Sgt Rock]

That may be true, but not what I was referencing. Has to do with barrel length.

Keep guessing...

[Bruce Collector]

On riot guns the barrel markings are stacked from the receiver forward.

On trench guns the markings are on both sides of the barrel so they can be seen with the heatshield installed.

[Chuckindenver]

my 520 and 620 trench guns have the markings left and right.
my 620 trainer, and riot have them on the same side.
also,
a trench will not have a hole drilled for a sight bead, as it is in the heatshield.

[Derrick R.]

Its the 3 asteriks. $ would be a full choke

[Sgt Rock]

Bingo! Derrick is correct...the 3 *** denote cylinder choke, the correct choke for a real riot.