This one's been discussed before, both on the 'Colt Forum' and in the 'Historical Society Forum'.
The Transverse Pin was first installed in 1877, on the Lightning.
The Flattop Model featured it in 1893 - at about serial number 152,800.
All Bisley Models have the Transverse Pin.
At about serial number 164,100 - in 1896 - all Models and variations of the Peacemaker were so equipped.
Most available references cite calibers and not barrel lengths, though early variants around the 1875-1876 timeframe have been so documented.
When you look at the collections, you'll see a preponderance of 5 1/2" barrelled revolvers.
The longer 7 1/2" Colts are seen almost as often, but the shorter ones seem to become more prevalent later in the Frontier era.
On the practical side, it's easier to hit consistently with longer-barrelled revolvers, and the 5 1/2" would seem to've been a nice compromise in accuracy and balance for the average man of the time.
Vaya,
Scouts Out!