Aldis Brothers Birmingham Unit Sight

Does anyone know what this sight was used for? solid brass with engraved rings around the body in 3 positions,, it's marked Aldis Brothers Birmingham, Unit Sight, No 73388, Eye End, it only measures 339mm long and is only 25mm diameter,, it has 2 circles etched into the optics the larger circle has the number 100 near it,, it's best viewed held away from the eye by about 6 inches,, any comments information appreiciated,, I'v seen these sights before but they are always bigger and normally 2" diameter not 1".