Bill,
I do not know if the hole in the 1961 big truck thermostat is original (or, if the thermostat itself is original).
The 1961 manual shows a Dole-brand thermostat with bypass system for bigger trucks (with 261 or V-8s). The by-pass system is separate plumbing and a special housing (not used on the school bus that provided my engine). The two extra hose holes in the school bus water pump (for the by-pass system) were plugged.
It looks like Stewart/Robertshaw and other thermostat companies sold thermostats with 1/8" holes already drilled into them.
