[rsk289]

Thanks Dave, I know what you mean!
The problem is that it's an early Smith's gauge, and they didn't have a hole for the bulb - instead, they have a kind of 'window' in the casing just behind the chrome rim at the front for a bulb to shine through. The bulb was held in a clip on the metal bracket that screws on the back of the gauge to hold it in the dash. This style of instrument doesn't have that bracket, it's just a snug push fit.