gunzik, some good info! Here is what i have managed to find over a few days of research.
1. The actual start production date is not carved in stone. Some sources list as early as 1870, some as late as 1875. I have seen photos of Colt advertising that was dated 1872 showing pictures of first Variation guns so some at least were made by then. All sources list 1912 as last year of production.
2. There are 3 (or 4) variations of the Colt 3rd model Derringer, depending on how you count variations. I will list them here as they are normally referred to by collectors.
1 Colt 3rd Model Derringer, First Variation,
Also called the "pregnant frame" model with the swivel bolster, high straight hammer, and short tightly curved grip. Serial #'s 1-2199 2,199 of this version produced.
2 Colt 3rd Model Derringer, Second Variation,
Same as above, with the high straight hammer and short tightly curved grip, but without the "pregnant frame" swivel bolster. Serial #'s 2200-24000 21,800 of this version produced
3 Colt 3rd Model Derringer, Transition Model (also called Third Variation Model by a very few collectors)
This has the sloped, slightly more curved hammer of the later production guns that come after it, but has the shorter tightly curled grips still. Not very common with Serial #'s 25000 -27000 2,000 of this version produced
4 Colt 3rd Model Derringer, Third Variation (also called Fourth Variation by a very few collectors)
This is the Colt 3rd model that most of us are familiar with, sloped hammer and more open curved grip. NOTE, serial numbers for these guns are all their own!!!! Serial #'s start at 1 again and go to 45000. 45,000 of this version produced
If the above numbers are correct, this makes a total production number of 70,999. This is very important information to know, as there will be (Very likely) 2 guns of the same serial number of most numbers up to 27,000. This becomes very important as we start looking at serial numbers spanning the 23,000 to 27,000 range as there "could" be guns with identical serial numbers some of which are antique and some of which are not!
Knowing for a fact from Colts own advertising that these guns were made at least as early as 1872 gives us a 40 year production window. If we divide our 70,999 guns over our 40 years we get about 1775 guns per year. 1775 guns per year x 12 years that are post 1897= 21,300 post 1897 guns, give or take of course. This gives us a serial number upper limit of about 23700 for antique status. This of course is all rough guestimation. If the links listed above are more substantiated, 28,000 would be a better upper limit.