Thought it might help some folks if they are looking for a suitable after-market horn for 78 or later 40s, 55s, and I believe 60 series. Stumbled on these hanging on a rack at AutoZone. They are made in Italy and cost about $16. A plug and play install on the 40...just slightly smaller in diameter and the sound is very much like the original. Here's some pics....Toyota horn is on the left.
(These will also work on pre 78s as there is a ground jumper wire included to ground the horn to the chassis.)
(These will also work on pre 78s as there is a ground jumper wire included to ground the horn to the chassis.)