The black and pink bike that eventually became the hill climb was weeping a bit when I acquired it at only two years old. I got a gasket for it at Motta in Italy on the Arcore and back in a week by Nordwest trip (on my other nordie).
I suspected a slightly warped head when it gave trouble as a 604.. Pressurisation of the cooling system is a classic symptom The coolant passages have not a lot of meat between them and the bore. Even less on a 604. I even got another head of Gary Rowe that had been surface ground because he had the same problem.
So not an unknown issue even on standard machines.
"Do not adjust your mind, there is a fault in the reality" 1957 175 Sport