I was in need of an early metal horn bracket for my 57. I guessed that the bracket wasn't exclusive to H-D and and looked somewhat familiar. I found one lot of brackets on eBay for cheap so I gave it a shot and Bingo! They fit exactly. I just needed to drill out the upper holes to fit bolts rather than the rivets. I believe these changed from metal to rubber during the 1957 production run although I see only one part number: 69013-54
I see I put the power pack on backwards but you get the idea. Is this how the brackets line up?