It is mounted on the bottom yoke, concealed in the headlight nacelle. The horns mounted on the frame are often the earlier, very heavy, Altette type. Yours is a small, lightweight horn, very similar to modern Japanese horns. Your bottom yoke should have two 1/4" BSCY threaded holes to accommodate the mounting bracket. Somebody will chime in with the actual Lucas part number for the horn (not sure if it's 6H?, I'm away from my reference material at the moment).