[OldMarksman]

No eye witness accounts, no written record, no pictoral record, no cylinder pouches in pics or museums.

How about this:

The Longest Ride

In 1907 Pony Bob described his experience on “the longest ride” to author William Lightfoot Visscher: “I had already ridden seventy-five miles [from Friday's Station, near Lake Tahoe]; but to my great astonishment, the other rider refused to go on. The superintendent, W.C. Marley, was at the station, but all his persuasion could not prevail on the rider, Johnson Richardson, to take the road. Turning then to me, Marley said: “‘Bob, I will give you $50 if you will make this ride.’ I replied, ‘I will go at once.’ Within ten minutes, when I had adjusted my Spencer rifle, which was a seven-shooter and my Colt’s revolver, with two cylinders ready for use in case of emergency, I started…I pushed on to Sand Springs, through an alkali bottom and sand hills, thirty miles farther, without a drop of water all along the route.”

And Phil Spangenberger offers this:

For irrefutable documentation of the practice, one can refer to a number of studies by respected firearms historians, R.L. Wilson, Roy Marcot and R. Bruce McDowell, who not only write about it, but also show numerous photos of revolvers produced with spare cylinders. The manufacture of revolvers with extra cylinders was not uncommon, with some revolver makers, like Remington, advertising the sale of extra cylinders.