Yes, looking down into the crankcase that's on the bike you can see that it is small journal, and that jive's with it being a '55 (learned by going back to Guy's earliest posts). However, the spare cases are from a later bike, as Guy said. The shape of the crank around the plug, and the size of the plug itself, says to me (am I "hearing things"?) that the spare is large journal.
About that plug, it almost looks like someone tried to weld it over or remove it by the welded-bolt method and failed. Guy, are you sure there is a threaded plug there?
Richard L.