olezippi,
This horn may not be aftermarket, but I did a quick search on eBay and I believe this is close to what is in your pictures.
Go to eBay and type in "Original 1947-53 Chevrolet GMC Pickup Truck Trumpet Horn 6V OEM Delco GM".
...and Tim, thanks for helping to confirm that the horn on my truck is most likely original to my truck. It is the "sea shell" type and located exactly as you described.
