Hi David, I've never been any authority on Speedmasters now even less as my Omega passion has waned considerably over the years, still as far as what you mention goes:
3570.50.00 is the watch reference, as you can spot on Omega's website at http://www.omegawatches.com/index.php?id=303
861 is the movement reference used by Omega, which is derived by Lemania 1873, which uses its own naming convetion as it used to supply different brands also outside the Swatch Group. 1873 is of course the naming also TH used.
Though minor changes have indeed taken place, most of them are cosmetic. So if you go for the hesalite crystal, steel back 'original' version (as it were) then this watch in the very basic features has not changed meaningfully in the past 30 years and this gives you considerable leeway as price range.
Cheers.
Fabrizio