You're conflating the term "stopwatch" with an all mechanical device. That wasn't even true at the time period specified-- the quartz movement had been created

You're conflating the term "stopwatch" with an all mechanical device. That wasn't even true at the time period specified-- the quartz movement had been created in the 1950s and the first quartz pocket watch had been introduced in the mid 1960s. The issues you bring up are only relevant to a particular set of devices that can be used as timepieces that measure the amount of time that elapses between its activation and deactivation-- and are are all stopwatches.