Those could have been retro fitted.
Type S horns with the bracket bent at a 90 degree angle were new for 58, 59 bracket was completely straight, 60 was straight but had a small step close to the horn. These horns had a large triangular trumpet and were the first model riveted shut. Around 61/62 they went smaller still called Type S but now the trumpet was square.
IIRC the parts catalog says the same LO horn was standard equipment on all Chevy pickups(possibly trucks) from 58-62 or 64? They were using the older design after it was obsoleted:
http://www.delcoremy.com/find-a-part/product-details/9000347Original location for the LO is on the drivers side second bolt down from top of radiator support. When I got my truck it had the original horn but I retrofitted a 4 note from a 70s Cadillac.
