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The large case with external bezel, Omega believed, would allow wearers to quicker reference the scale for measuring speed.Rolex's Daytona – born a good six years later in 1963 – was to borrow this concept by putting a tachymeter scale on an external bezel, as did Heuer with its Autavia.All backs of the 2915--2 feature a single bevel to the edge. This watch was made for only one year –1959 – and is the very definition of the "transitional" watch.
I've seen 2915-3's with a steel bezel and alpha hands; I've seen them with a black bezel and broad arrow hands.
Launch in 1957 with reference 2915-1, the entire reference, including 2915-2 and the transitional 2915-3, would last barely three years.
Identifying a 2915 watch is perhaps the easiest of all Speedmasters, due to some very noticeable differences from later watches, not the least of which are the broad arrow hands. Consider that at the time, a tachymeter scale existed – at best – on the outer rim of chronograph's dial.
The Heuer Carrera, however, would not receive a tacyhmeter scale at all until the mid to late 1960s with its 2447 reference NST and SNT – the "T" denoting tachymeter scale.
Still, these scales were printed on the dial, not a bezel.