The Dynabeats were likely all produced from the early to mid 70's. Timex increased their beats per hour to 28,800 instead of 21,600. The positional time keeping of serviced ones are usually excellent on the timing machine yet not as good when off the wrist. Now, as a rule, the higher the number of vibrations per hour the more accurate the watch;however, this is not necessarly the case with the Timex Dynabeats, but they are still good timekeepers. Timex Electrics are cool watches.