"Those holes , particularly the ones close together in the middle, are from a 'Field instruments' M mount. Have one on my Martini Hornet. Will post a photo later when I get a chance.

The other holes, no idea, but mine has the same pattern.

Chrispy"

So you need to find a Field Instruments M Mount. (like Sarg has said).

Though apparently they can also be easily made from a piece of angle iron.