The serial number. I don't know any sites off hand, but I've seen charts of how to date a Speedy based on that. If you want to be specific, you can send a request to Omega and they will tell you the manufacture date and the location it was first shipped.
Here's the problem with using any of the various "movement charts".
Every Speedy Pro has a unique number on the escape wheel bridge. (On later model Speedy Pros, this number is also engraved on the back of the 7:00 lug.) Most refer to this as the movement number but more accurately, this number should be called the "escape wheel bridge" number. As they are machined and engraved, these individually numbered bridges are collectively placed in parts bins awaiting assembly into the movements.
During the assembly process, these numbered bridges are randomly selected from the bins and installed along with the other parts. No one takes the time to remove them from the bins in order; i.e., first in, first out. Note that this selection process might permit a bridge to sit in a bin for years before being used.
Once assembled, the movements are "put on the shelf" for later installation in the cases. Again these movements are randomly pulled for installation in the cases. No one takes the time or the effort to insure "first in, first out". It may be some time (even years) before an assembled movement is installed in a case.
Obviously, the parts bin may contain a mixture of bridges produced over the years a particular movement was produced. For example, the cal. 1863 has been in production since the mid-1990s. The parts bin could conceivably contain bridges manufactured and numbered in 1996, or any year since, creating the possibility that a movement assembled in 2008 might have a bridge number dating from 1996. Likewise, a movement assembled in 1996 could sit on the shelf for years until being installed in a case in 2008.
I'm not a mathematician but the probability of the movement/bridge numbers providing any useful (i.e., accurate) information regarding date of manufacture is negligible in my opinion.
So how do we really date a Speedy Pro? Is it the date the bridge was machined and engraved, the date the movement was assembled, the date the watch was assembled, the date the watch left Bienne, or the date it was sold at retail?
If you buy a new Speedy Pro (which I have never done and do not recommend anyone to do), just use the date on your receipt and enjoy.
If you buy pre-owned, don't even worry about dating your Speedy. Your "precious" will give you years of reliable service regardless of its age. (My 105.012 Speedy recently returned from its first repair, a "mainspring transplant", after 40+ years of enjoyment.)
Note that some use the date engraved inside the caseback of vintage Speedies (i.e., 105.012-65) as the watch's year of manufacture. Again, the "part's bin" factor could result in a "65" case back being used in the assembly of a watch in 1969.
As Mercuttio suggests, contact Omega and pay for an abstract.
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