Not sure why, but while you are waiting for Chris P to respond you can look up a 15020 Evinrude 18hp under a 1957 7.5hp on MarineEngine.com, not sure why it is out of place, but I found it the other night looking for a head gasket number. I know it’s not your 15, but like Richard said the gearcase is very simular. Hope that helps. Chris P will have all the particulars.
Dan in TN