My 2004 Sierra catalog lists Sierra #18-2952 as the head gasket for 1957-58 7-1/2 HP. It might be the same for 1955 , as these motors didn’t change much back then (maybe someone here knows?). Says it replaces 203034. See if it looks like this one: https://www.marineengine.com/newparts/part_search.php?part_num=203034
If you call marineengine, they will probably know for sure.
Dave
