Triple Polstens on a towed mount

[Hanno Spoelstra]

I think this is another truck mounted one: "A British triple 20mm anti-aircraft gun position guards the Bailey Bridge at Arnhem, 18 April 1945."
IWM BU 4138.jpg
Source: https://www.iwm.org.uk/collections/i...ject/205207121


Your second photo of one on a towed mount: "Mobile triple 20mm Oerlikon gun during an anti-aircraft artillery demonstration at Sheerness, 2 July 1943."
IWM H 31130.jpg
Source: https://www.iwm.org.uk/collections/i...ject/205199053


And another tank chassis mounted one: "Crusader AA tank mounting a triple Oerlikon gun in a hull-down position, 19 July 1944."

Note that these guns are often erroneously called Oerlikons - why would that be?
IWM B 7738.jpg
Source: https://www.iwm.org.uk/collections/i...ject/205206079