My theory has been a little different. IIRC Horn and Please are written on the two sides asking the tailing vehicle to blow the Horn. And the OK comes from the OK word written on the bumper (in the middle) by the manufacturer. This was a requirement as far as back in the 80s (may be even earlier) to stamp the OK (on the lines of OK TESTED) I guess. People got habituated into reading Horn OK Please and the integrated with the culture.