The link below is to a photograph in the IWM archive of Capt. (later Major) Charles Henry Greville who served with the 1st Battalion Grenadier Guards in WW1.
https://www.iwm.org.uk/collections/i...ject/205295817
The cap badge he is wearing is presumably a small OSD, about 25 mm high (when measured against the buttons which are about 23 mm in diameter).
Does anyone have an example of this badge that they can show please?
Bill