Hi Ace
Welcome to my dissertation, The 1866. I doubt anyone (even Uberti) could give yo a definitive date when ALL the 1866s changed. Uberti built the early guns right along side the "change model until they ran out of parts. One version of the early guns has a trigger block safety, one doesn't.
The early guns had a much shorter Carrier Block, Breach Block (bolt), firing pin, firing pin return spring and smaller link pivot pins. I never paid much attention to the hammer or the firing pin extension rod, although the extension rod is a different diameter. The cartridge return ramp on the carrier was much steeper than today's guns and the guns were very very cartridge length sensitive. The lever side springs were also of different design.
There are absolutely NO PARTS available for these guns. If you break something and you don't know a great machinist/gunsmith, you have a Great Wall hanger.
These guns were manufactured about 20 years ago and earlier.
The next series continues with some upgrades and refinements right to today's guns. In the current series, the early guns had the firing pin extension rod retained by an itty bitty cross pin. In the instance of an out of battery discharge, the gun slamming open bent the lever and sheared the itty bitty cross pin and the only thing to stop the extension rod from exiting the action was your face. Uberti increased the size of the cross pin .... No definitive date when that happened, but about 6-8 years ago.
Then Uberti up-graded the Ladle (loading gate). They (Uberti) took a ladle that never broke and upgraded it to a Ladle that is guaranteed to break. They still use that ladle. It needs to be replaced immediately after purchase with a after-market ladle and then reinforce the little tab on the after-marked one. At this SA me time frame Uberti changed from a small head firing pin to a large head firing pin for more reliable ignition.
The next change, about 4 years ago, was to redesign the retention of the firing pin extension rod. This required a different Breach Block and a different extension rod. The new version is bullet proof. Uberti DOES NOT support the old cross pin retention method with parts. If you break something you have to purchase a "kit" that includes the new Breach Block, Extension Rod, Firing Pin and return spring.
There you have it. 1866 101. Best bet is to purchase an 1866 manufactured in the last four years. Manufacture date codes are available.
Coffinmaker