Polsten
Polsten is an anti-aircraft gun or autocannon.
Also recorded as Polsten 20mm.
In detail
It is associated with United Kingdom.
Sources
Each source is named with the standing the house places on it, so the evidence can be weighed rather than taken on trust.
reference work · Reputable secondary · Wikipedia
“20 mm Polsten”, English Wikipedia, consulted as further reading
Consult the sourcereference work · Reputable secondary · Wikidata
Wikidata, structured authority record Q739637: Polsten
Consult the source