While this is not an answer that should be taken with any facts to base it on, I would think that the "ending" to the model numbers could indicate "batches" during manufacturing, or where the device was assembled. Another thought I have is when manufacturing another "batch" some piece of hardware changed for what ever reason, even if they used a newer chip for something like the the GSM radio.
I also looked up the SGH-T959V (The 4G version of the Vibrant) and they only have 1 "sub-model", which this device did not sell very well, and I have not heard of any "upgrades" to it.
Another example I could use to support minor hardware changes is these devices have, as you probably know, bad GPS chips. Samsung has made attempts to correct this while the device was being manufactured.
My device does not even have ANY of those model numbers anywhere on it. All mine says is SHG-T959.